I would have to go for under siege 1, because it is the closest thing to a real movie, or maybe the first one, whatever that was, because of the cool dojo scene.
The Path Beyond Though had way too much butt kissing for my taste, but the techniques are quite good, and it is interesting to ponder why he was balder in 1970 than he is today! In any case, if Seagal Sensei put out a straight instructional video without the ego inflation, I would snap it up.
Interesting enough that everybody refers to him as Seagal, like he is the older kid on the block instead of say Seagal Shihan, or Seagal Sensei... Guess the movies have had an impact there.
Incidentally, i saw him on a cooking show on TV, Japanese TV. Not only is his Japanese very very good, but when lined up with a 120 lb japanese comedian, he provides a great straight man. And also has good taste in food!! Would be worth a laugh if you could see it on the net. He lays a poetic pick up line on the female hostess!