What Castlevania game do you recommend?
Depends on what you're looking for. To be honest, outside of the 3D ones, there really isn't a BAD Castlevania game. The Classic series can be very difficult, but OH SO satisfying.
Honestly, if at all possible, I recommend the Dracula X Chronicles for PSP, as you get the best of both worlds for it. Not only is it a remake of Rondo of Blood, but it includes the original Rondo of Blood fully translated and dubbed with the english voice actors, AND also has the entirety of Symphony of the Night on it, also with updated VAs and some bonus content, such as a more faithful Maria Renard playable (She was playable in the Saturn Only version of SOTN, but played quite different from her other incarnations. I think the PSP even includes some of the exclusive areas from the Saturn version, don't quite remember.) So it gives you two of the best games of both sides of the spectrum in one package.
So Dracula X is a great way to try both ends. If you like Rondo, try the other classic games. If you like SOTN, give the DS Castlevania games a go. Portrait of Ruin being my personal favorite (and also including a mode that lets you play as Richter and Maria, fittingly enough)
If you like both, just play ALL of them, because most of em are pretty great.
Aria/Dawn of Sorrow and Symphony of the Night. If you like those you can move on to other games
Aria and Dawn are horrible starting points, imo. Not horrible games, quite the opposite, they're both fantastic, but they both really rely on you having at least a decent degree of prior knowledge of previous games in the series to get the most out of it. Especially with its twists. Not to mention it having a much different tone from the rest of the games thanks to the time period it takes place in. The Sorrow games are best saved for when you are more familiar with the rest of the series, I feel.
SOTN is helped by having played both Rondo and Castlevania 3 as well, to be fair, but I also feel it is self contained enough to be fine playing it before the others.