I haven't seen all of his films, but there is little doubt in my mind.
Under siege 1 is the best picture he's ever made. Tommy Lee Jones is GREAT as the semi-psycotic ex-CIA. The other bad guy is really great too (his name escapes me at the moment), and the girl playing the part of the playmate has got some outstanding qualities.
Under siege 2 is definately the best comedy Seagal has done. It's so macho and unrealistic that I can't watch it as anything but a hilarious comedy. The part where he runs through the train as it falls allmost vertically down from the bridge and narrowly escapes through the door in the end of the train - all without even looking a bit tired or even getting his clothes dirty is such a laugh. I don't know if it was intended to be a funny movie, but I think it is. The 'token-negro' and his shake-and-bake nikyo mentioned in an above post was a great scene too.
Let's face it. Seagal might be a great martial artist - I have never seen him 'for real' so I don't know, but hi's definately not the best candidate for an Oscar in my book.