The thing that's stopping me from picking any of them up is I have never played the series and therefore have no nostalgia for it. While I've heard they're quite good I don't even know what type of game they are besides 3d Action/Adventure, and I'm not sure how they've aged. I may still grab Soul Reaver 1 with my leftover blue coins, to test out the series as an essentially free game from the Bethesda bundle purchase.
They have a really good over-arching storyline and were well regarded when they were originally released. However, without the nostalgia factor, I'm not sure if they still stand up on their own. Soul Reaver 1 was originally released for the PS1 (and later the Dreamcast and PC). Soul Reaver 2 was released for the PS2 pretty early in it's lifecycle.
Soul Reaver has great puzzle elements, and plays a lot like the earlier Tomb Raider games (they are, of course, made by the same folks). The problem is, it also looks like an early Tomb Raider game.
Even though it's the "end" of the story, I'd almost suggest starting with Defiance if you're only casually interested. There are two characters, one that is heavier on the action, and one that is heavier on the puzzling. There is also some pretty awesome voice acting, which is also in the earlier games but much more limited due to the technical limitations of the PS1.
Just my $0.02