I can't choose.... I love fine art museums, Philadelphia Museum of Art is great, as is the Rodin museum nearby. Yay
The Thinker. I've been to the
Barnes Foundation twice, and it's incredible.
Then again, though... MoMA was a lot of fun, the short while I was there. I like modern-type stuff a lot as well.
The Franklin Institute is a lot of fun, but growing up around here means I've been there a gazillion times, for school related stuff and otherwise. And Academy of Natural Sciences, dig that as well.
However, I love history stuff as well, especially with all the American Revolution type stuff around Philly.
The National Constitution Center is fairly new, and is a really cool place to visit too. There's an exhibit on Ben Franklin that just opened, I want to go see it over winter break. Independence Hall and Franklin Court are also cool. Not in Philly, but Washington's Crossing was also awesome.
....I think I just like a lot of museums. :)