You should play them in the order they were released. I think you'll appreciate each game more that way.
Not a bad suggestion.
Crazy Eight - If you're wanting to play the ClassicVanias as well as the MetroidVanias and the
only one you've played so far is SCIV, my most important piece of advice to you would be prepare to die,
a lot. SCVIV is almost certainly the easiest old school ClassicVania. Part of the fun of the older games is their insane difficulty. You
will die on many screens until you absolutely master them. Classicvanias are at their best an extreme test of your reflexes, your memory, your attention and your patience. It's a triumphant old school feeling every time you beat an area you were stuck on, to say nothing of finally beating the whole game.
My recommendations are-
Symphony of the Night (PS1)
Circle of the Moon (GBA)
Castlevania (NES)
Castlevania III (NES)
Those should be good for a start.
I think "Legacy of Darkness" is probably the best 3D game, but it is an N64 exclusive and difficult to find, especially if you want the box and manual. I once saw a boxed copy with the strategy guide sell for over $100 on ebay, no joke. Though I waited and shopped around and got them both for much less.
As for other rare games, Rondo of Blood (Japan only PC Engine game) Dracula X for the Super Nintendo and Castlevania Legends are the most notable ones. Dracula X is a quasi-remake/port of Rondo for the Super Nintendo. Rondo is pretty universally considered the better game though, being a popular contender for "Best Classicvania". "Symphony of the Night", probably most often considered the "Best Metroidvania" in the series, is a direct sequel to Rondo by the way. Both Rondo and Symphony are available on the PSP "Dracula X Chronicles" but are better served by a full sized screen imo.
Legends was the first instance of a lead/main female protagonist in the series, as well as the canon origin story before "Lament of Innocence" came along. Said origin is somewhat controversial as it implies that the Belmonts are descended from Dracula by way of Alucard, which I think is a great idea but ymmv.