Let's break this down by game, then.
Lament:
Not Dracula yet at all. This is where the connection with Chaos starts, though....
Legends:
Not "The Dark Lord" yet. Only kinda starting. Just a Vamp pissed off at the world for killing his SECOND loving wife (poor guy can't catch a break).
CVIII:
Risen as "The Dark Lord" due to Necromancy and other dark arts. Truly at Full Power. However, was fought by a team of Four, not just one (though gameplay-wise you only fight him with two). <----here's one instance, methinks.
Curse of Darkness:
Not at all Full Power. Recently resurrected when Isaac does his thing and Trevor does his thing with the blood and the stuff.
Adventure:
Possibly at Full Power. Fakes his own death.
Belmonts' Revenge:
Possibly at Full Power. More power than in Adventure, unless defeat at the hands of Christopher left him weak and regenerating.
Order of Shadows:
Not at all at full Power.
Castlevania:
At full Power. Enough power to put a curse on Simon upon dying. <----here's one instance
Simon's Quest:
No power at all. Barely a regenerating Skeleton. Simon whips his ass.
Harmony of Dissonance:
Not even with a body yet. You fight a ghost, and then an amalgamation of parts before they have a chance to unify and do anything.
Rondo of Blood:
Possible candidate for "Full Power", given the blood ritual... but the kidnapping of maidens suggests that he's still recovering. I think this one might be a candidate for it, though <----here's another possible time.
Symphony of the Night:
Recently defeated. Only a few years after Rondo. You fight Dracula in an ethereal Castle that's not of this world, not unlike in Harmony.
Circle of the Moon:
He states he's not at full power and requires a ritual of the moon and Morris Baldwin's sacrifice.
Order of Ecclesia:
Not at full Power (unless you count the Bad Ending as such). Barlowe is a poor sacrifice for this ritual.
Legacy of Darkness:
Not at full Power. However, the ending suggests that, in time, and using the power that Cornell relinquished in order to save Ada, will help him become stronger in time.
Castlevania64:
Possibly at Full Power, given the events of LoD, the 8-year gap, and the countless children Actrise has sacrificed to Dracula. It's possible that Gilles DeRais's perversions have also helped the Count be at full power.
Only problem is, he's been revived in the body of a child. But gameplay-wise, he has enough power to make you fight an impostor, then make you fight his human form, and then make you fight his Dragon form... and what a form THAT is.
It's possible that Reinhardt and Carrie fought Drac together... but since the game is not Canon, it is unclear how they went about it, as there are no references to these heroes at all later.
Dracula (the Book):
Dracula is purchasing property and is able to travel and he's able to use all of his powers. It takes quite a lot of work for Van Helsing and company to take him down. This nullifies Bloodlines and PoR from being candidates for "Full Power Drac", in my opinion... since Drac dies in the book.
Bloodlines: Because of Dracula (the book), and because of Drolta's schemes, and Elizabeth Bartley/Bathory's plot to use the blood of maidens, etc. to resurrect the Count, we know that he's not at Full Power in this battle.
PoR: Same reasons as per Dracula (the book) and due to the events of Bloodlines. On top of this, you fight Drac and he requires Death's assistance. He's also sealed within the magic of Brauner's paintings for some time. Obviously weak...
...but the deaths of many during World War II may be why he's so young-looking. Interesting.. hm...
1999: At the strongest, but this is only referenced later. We do not know the extent of Drac's power, but we know it took an army, the Church, and other secret organizations, combined with magical rituals from both religious and secular groups, to defeat Dracula and move his realm to a place where he couldn't attack anymore and his connection to Chaos is severed. Dracula has been linked to Chaos (a demonic entity... possibly the biggest demonic entity... perhaps even bigger than Lucifer... just the embodiment of Entropy and Disarray... we're talking Eldrich Abomination, here) since Lament and the Crimson Stone.
AoS: lose to Chaos and 'technically' you're Drac on his prime. Not the canon ending, though.
DoS: lose your mind and 'technically' you're Drac reborn, resurrected, and on your strongest prime. It will take a team of three (Grant's representative got snubbed) to take care of Somacula. <----here's another candidate for "Full Power" but not a canon instance.