Which one of these is best value for your money? Never played an Animal Crossing before.
I'm trying to think of something to compare Animal Crossing to...the sims? minecraft? If you like that type of casual gaming, AC will last you years. It's a game you would most likely download to keep on your 3DS so you can play a bit each day. In terms of quality it is a Nintendo AAA. I'm sure Castlevania and specially Luigi's Mansion will be quality experiences but they are more linear/traditional games so they will come to an end.
Animal Crossing has no end game. It goes on forever...so depends if you like closure or not.
If you're talking about 3DS games, Luigi's Mansion: Dark Moon is pretty fantastic. It took me about 10-15 hours to play through, though, and there's not much replayability (unless you're the completionist sort), so if you're looking for something that'll hold your attention a bit longer it'd probably be the new 3DS Animal Crossing.
The 3DS Castlevania has received middling reviews at best, and I've heard only negative things from friends who bought the game.
I'm sure Animal Crossing is the one you could get the most out of assuming you like that kind of thing. Luigi's Mansion is pretty long, much longer than it needs to be with tons of stuff to do and Castlevania is about 8-10 hours without any good reason to ever return to it after it ends.
Thanks for the replies all! Normally I like games that actually end at some point (one with a plot/''purpose'') but have been enjoying games like the Atelier series/Monster hunter (not really the same) and the social aspect of Persona. Was also a big Harvest Moon nut back in the day. Maybe I should grab a free AC then? Would be either that one or LM