Well, basically, if you didn't grow up playing games in the '80s to mid-'90s, the difficulty and structure are going to seem rather extreme with many of the older titles. Moreover, the thing is, Castlevania itself has shifted and morphed from what it originally was over the years. At its start and at its core, "Castlevania" was a series about environmental strategy, timing, consequential platforming, and trial & error, whereas it has turned into a series marked more by agility, muscle, super powers, and meandering. (Not that some of the latter titles don't have their points, but the philosophy of the design has changed, which disappoints some who loved Castlevania for what it was: creative and challenging action-platforming). Bottom line is, some of the best entries require you to fail again and again, to learn and have a rewarding experience. No offense meant by any of this, but modern gaming is generally more about getting something over with and instant gratification more often than not.
With that out of the way, I think you'll have the best luck with the following, IN THIS ORDER:
Super Castlevania IV (Easily in the top 3 games in the series, and arguably the best. It plays like Castlevania/Castlevania III at its core, but with far more forgiving and dynamic controls--including the ability to change direction in mid-jump, crawl, swing across gaps, use the whip as nunchucks, a shield, or a dangling trap, and/or whip in 8 directions. In addition, it features amazing level design, atmosphere, music, and special effects. It's a must play!).
Castlevania: Bloodlines (Along similar lines as "Super CV IV," but with a different visual style and cool quirks of its own).
Castlevania: Dracula X *or Dracula XX*(This complete reimagining of "Rondo of Blood" takes the basics of Rondo but leans more toward the earlier games in the series in terms of challenge and design. It features a neat new watercolor/comic-esque art style with new special effects, and it has a more consequential/rewarding use of branching paths, keys, and hostage rescues than Rondo. Some, including myself, feel it is a more enjoyable game than Rondo, despite having less scope overall).
Castlevania: The Adventure Rebirth (A Wiiware total reimagining of an early Original Gameboy title that can be simple and fairly modern in its normal modes, or extremely difficult on its harder settings).
Castlevania II: Belmont's Revenge (A direct sequel to the Original Gameboy's "The Castlevania Adventure" game that features a level select like Mega Man and is a fan favorite. It gets pretty hard in later stages, but it is still considered fairly accessible on the whole).
AND:
OVERVIEW on other "classic" CV games:
Castlevania (The original game is solid, but can be unforgiving and rather plain. It was a good start, and there's nothing really that wrong with it, but it isn't among the best, IMO, despite laying down a great formula).
Castlevania II: Simon's Quest (This is the proto-SotN. It has a great mix of the classic style's dangers with the adventure elements of the latter games. Once you learn the basics, it's a pretty manageable and memorable entry. Some obscure puzzles involving magic crystals may give you trouble, though).
Castlevania III: Dracula's Curse (This is definitely among the top 3 games in the series for its diversity of levels--structurally and visually--and its deep game system of multi-characters and the best branching paths map in the series. If the Clocktower is giving you trouble--while Grant is very useful to add to your party--you can always choose a different route. With all the possible combination of routes and the progress shown on the shifting map, you really feel like you're on a journey in this game--and it's exciting to see where you'll wind up next and what challenges await. I'd stick with it, or at least go back to it after you've played "Super CV IV" and other classics that might ease you in better. While CV#1 is solid, and CV#2 is very interesting, this one, CV#3, really is exceptional and not to be missed. And also, with these old titles, sometimes it's not about beating the whole thing, as it is experiencing the journey and getting as close to the Dracula boss battle as possible on a given series of attempts; then returning later on and increasing your mastery and progress until you do overcome the Count. CV#3 is definitely worth finishing, though).
The Castlevania Adventure (This one on Original Gameboy is for only the most hardcore classic Castlevania fans. While it introduced the flame-shooting whip, it lacks all sub-weapons, features the most sluggish controls in the series, and has a very demanding level design. Still, there's something charming about its challenge and unique visuals/enemy set. Regardless, with where you're coming from, it's wise to stay away from this game until you've gotten through/tried all the other classics)
Castlevania Chronicles *or Castlevania X68000* (A reimagining of the original CV game. Not really worth the time, IMO. While there are some interesting visuals and ideas, this one is pretty mundane, even for a fan of classic CV like myself. EDIT: However, if you want a visual update that is more in line with the first Castlevania, with some unique twists of its own and improved controls, it might be worth a shot. It was created for Sharp Computer/then PS1 in the same way DXC was made for PC Engine/then PSP, and has original and arranged modes).
Castlevania: Legends (The third Original Gameboy game has a cool female lead, and it tries to be special, but just feels unpolished and boring, by and large. Very forgettable, unfortunately).
In SUMMARY:
If someone said they could only play through a handful of 2D CV games, I think I'd say eventually play:
Super CV IV, III, Dracula X, Bloodlines, and II (Simon's Quest).
(You already played Rondo...though I have mixed feelings about that popular entry).
Now, if you were to take the pulse of the this whole board, I'd guess that Super Castlevania IV, Symphony of the Night, Rondo of Blood, and Castlevania III: Dracula's Curse are considered the gold standard bearers. (Though, that certainly doesn't mean others aren't worth playing).
EDIT:
Oh, later on, if you haven't played Castlevania 64 or Castlevania: Legacy of Darkness, those are worth going through. The best 3D Castelvania games to date, IMO.
They can be rough around the edges, but those Nintendo 64 games had a lot of positive/innovative features that served up CV in 3D:
*Multiple unique characters (up to four in LoD, two in CV64) with alternate levels/bosses
*Full 3D gameplay (ala Mario 64, so it's not "on-rails")
*Multiple endings based on performance
*Level Design features spatial depth (vertical and horizontal--not flat hallways)
*day-and-night cycles with time-sensitive events (like Simon's Quest)
*Weather effects (rain, lightning, moving clouds, and "fog"--the last one likely being a graphical shortcoming that actually helped)
*Dynamic, real-time lighting (next to candles, for instance)
*spot-on atmosphere
*death-defying platforming of all sorts (including ledge grabbing).
*environmental/enemy hazards (medusa heads, spikes, guillotines, buzz-saws, cannons).
*innovative survival-horror/suspense elements
*Vampires that pretend to be human and vampires as regular enemies besides bosses
*status changes, including poison and vampirism
*manageable questing with inventory items (meat, keys, cards, cure ampules, etc)
*interesting, involving plot (characters like Rosa, Vincent, Renon, Malice, Henry, etc)
*3D in-game cinemas
*Some voicing
*Unlockable alternate costumes
*Long and short-ranged attacks (IE: whip + sword)
*upgradable sub-weapons (in LoD)
*There is a useful slide and duck/crawl play mechanic
*Diverse mix of old and new enemies in 3D
(As an aside, it's worth noting that certain Beta video elements didn't get into either game like swinging over gaps with the whip; but it shows that minds were in the right place)
EDIT 2:
Simon's Quest help info: Towns are for gathering information (some of it misleading on purpose), getting healed in the church, or buying/trading/finding items like in the Legend of Zelda. The Night and Day cycle is for multiple reasons. "What a Horrible Night for a Curse" refers to the fact that Simon Belmont is cursed and is slowly dying as he's on this quest, and when it turns nighttime, the enemies become stronger in mansions and outside, the towns are no longer safe havens (filled with ghouls), and certain tougher enemies appear. Also, certain events happen only at night. There is a special dagger weapon that can only be procured at night in a graveyard by making a certain exchange--an info piece you find by questioning townspeople in a certain town [Facts are different than I remembered; I guess only the N64 titles have night-sensitive events. The clues, while they can come from townspeople, don't appear to be the source for the particular event I was referencing]. There are no Save Points; instead, there are passwords you see when you die that you can use to keep your progress. You can level up by collecting enough hearts (which are also currency), though it's not obvious because there are no messages to tell you you've leveled up; though I think the life bar gets bigger. Finally what you are doing is searching out Mansions, where you need to get Dracula's Body Parts (eye, rib, etc) to burn to lift the death curse on Simon, and you need a Stake in each Mansion to open up the hiding place of the Body Part. The Body Parts also give you new abilities, too--like seeing hidden passages or blocking enemy attacks.