^ You got the best one. There's so much albums to get that I can't pick one or two. But you can try the last two ones, Deadwing & Fear of a Blank Planet, if you like the metal parts on In Absentia. You have Stupid Dream which represents the pop-rock period of the band (best albums for fans, but highly not recommanded IMO), and a lot of progressive stuff between 89-99. You can seek Stars Die - The Delerium Years, which is a collective of that period, and Warszawa, their most impressive live album.
But if you want a personal advice, get first their DVD Arriving Somewhere... , which is probably the best dvd I own.
