I voted for the art exhibit. I love the aquarium, too, but the art collection is more challenging to complete. At least, it is for me! Plus, I've taken a zillion Art History classes, so I enjoy reading the facts about each piece. There's something so cute about seeing a villager admiring a painting.
Bugs. It's cool to see them actually doing something. Rolling a brown ball. Wrestling. Working(the ants). Although avoiding the mosquito can be a pain.
My favorite is the art exhibit; I wish there were more artwork to collect and that as home decoration, the paintings were as big as they are in the museum. I do like the museum's aquarium too and I used to think it was my favorite exhibit; I forgot why that changed. I love history and any little bit of references to history in ACNL I really love (of course, I wish it was more diverse and had more things that I could collect for the museum; maybe have books, artifacts in a future AC game?).