It's completely subjective, and all the games are pretty decent, but I just liked the ambiance of Contracts because of how dark and dreary it was. To me, both Silent Assassin and Blood Money looked really cartoony/comical, and Contracts was a few shades grittier. I will admit that Codename: 47 was just too difficult to get through due to graphics (I know, what kind of shallow gamer am I).
Silent Assassin, at least to me, had a bit more of a coherent storyline as it was linear. Blood Money sort of ended up that way, however the storyline was at least for me, impossible to follow or decipher.
Contracts is more just a series of flashbacks that 47 has, but they are moody, dingy and dark, and have the feeling like you're actually the assassin. Again, in my opinion.
It's been a while since I've played it, so it may be shorter than BM or SA in level number, however it garnered alot more replayability for me.
Graphically, I think it looks better than SA for it's grittier, less cartoonish presentation.
As far as AI, you should be fine as long as you crank up the difficulty as applicable.
Good luck, you can't go wrong with any of them!