I'm not sure what you mean by "boastful" as he rarely sings about himself, and even when the narrator does it's not usually in a positive manner. So here are some songs that I would describe as "confident" and/or "shouty". In no particular order:
Commandante
This Year
Letter from Belgium
Going to Georgia
Going to Scotland
Jaipur
Baboon
Prana Ferox (just because it rocks)
Genesis 19-1-2
New Chevrolet in Flames
Love Cuts the Strings
The Best Ever Death Metal Band in Denton
No Children
See America Right
Oceanographer's Choice
Against Pollution (what's more boastful than bragging about shooting someone in the face with a shotgun?)
Heretic Pride (what's more boastful than bragging about being burned at the stake?)
Lovecraft in Brooklyn
Psalms 40:2 (what's more boastful than bragging about burning down a church?)
Estate Sale Sign
(Man, I just skipped through pretty much the whole discography. That was a lot of fun. Now I need to listen to half those albums all the way through.)