Hmmm... Deadpool...
Let's see, he's one of the characters in the straight-to-DVD movie Hulk vs Wolverine. While he isn't heavily featured in it, I think this is the best outing he's had in any non-comic book form.
He was also quite fun to play in the beta for the Diablo-style MMO, Marvel Heroes. Course, it is an F2P game, so you'll have to buy him or hope he drops from a boss.
I suppose you could also read the comics. If you're looking for the comical badass that we know and love today, start with the second Numbered series he had (2008-2012). Any Deadpool title before that is from the 90's and thus a little too serious and depressing. Well, maybe not Cable & Deadpool, but I haven't read that series, so yeah...
For something that requires no back story what so ever, Deadpool Team-Up was a fun little series from 2010. Each issue features Deadpool joining forces with a different superhero (mostly ones you've never heard of, but there are some bigger names, like Thor and The Thing). Don't get confused, though. Although the order you read them in doesn't matter, it starts with issue #899 and counts backwards from there.
He was also good in Uncanny X-Force Vol 1 (2010-2012), and Deadpool Kills the Marvel Universe (and the follow up, Deadpool: Killustrated) is one of my favorite comic, but these comics are a bit more serious in tone and nature.
