Everyone vote for Sin & Punishment for Retroactive! I know I did!
All three of these games are tempting to vote for. On the one hand, Castlevania 3 is some great stuff and I can fillibuster about it, but I'd much, MUCH rather play another game in that franchise because ffffffffffuck that game can be kinda rough when you have not played it in a while. and really, if we're going to look at any Classicvania title, I'd rather talk about both of the Richter Belmont games (Rondo of Blood and Dracula XX/Vampire's Kiss/**** port of good game) in a retroactive double header, or talk about the weirdness that is Castlevania The Adventure: Rebirth. or Order of Ecclesia.
And then there's Sin and Punishment. as Johnny said, it's a game not in the conversation at the moment. While I love treasure, I'd take about 8 or 9 of their other games* before I'd take Sin and Punishment's nonsense story and outright clumsy control scheme. I don't really like the game a ton, but hearing the conversation in the forums as I blast out crumudgeonry could be fun...
Nay, though... My vote is on Pokemon Trading Card Game for the Game Boy Color/3DS VC! thsi game brought me HOURS of joy. I don't know what it is about friggin' TCG portable games, but I eat them up like nuts. it beats the crap out of trying to scrounge up such a variety of opponents by having to actually go to an event only to get destroyed by some rediculously stupid troll deck in the format like Power of One or Night March. (I briefly started trying to get back into playing Pokemon TCG. tried a Donphan deck. eh.)
My fondest memories of the pokemon TCG come from the period of time unto which this video game encapsulates; a time where the game was in it's infancy and- you know what? I should save it for retroactive, because I hope you all vote for it too. in a way, it is an RPG with a non-linear progression that isn't just generic EXP.
*Gunstar Heroes, Gunstar Super Heroes, Dynamite Heady, Mischief Makers, Guardian Heroes, Silhouette Mirage, Ikaruga, Gradius V, Wario World.