tough call!
The Soft Bulletin is a very popular one, i think that's the album most would suggest.
but
Yoshimi Battles the Pink Robots my favorite, i think it's a great place to start. things get crazier the further back you go, so don't expect it to all be so melodic
another good one, maybe a more realistic approximation of their style, is
Transmissions from the Satellite Heart. some really great stuff on there.
i have their full discography including (almost) all singles, but i can't say i've listened to half of it yet! i'm just a bit compulsive about collecting...