I'd recommend picking up Soul Reaver far before either of the Blood Omen games. Then I'd suggest playing SR2, and then Defiance. The two Blood Omen games are definitely cool, but the SR games are just much better.

The reason Blood Omen 2 is so disappointing is based around few key factors:

1) Extreme technical issues. Blood Omen 2 features a sacreligious lack of polish. The hit-detection sucks, the framrate drops constantly and there are a number of clipping problems. On top of that, there are a number of cinematics in the game where the audio track is offset from the video by a full five seconds. Plus, the animations are stiff and repetitive and the sound effects are occasionally glitchy. There's just a ton of bugs.

2) The puzzle design is atrocious. Seriously, it's all lever and block puzzles. Boring, uninspired and very, very done.

3) The level design is 100% linear. You don't revisit areas, there's little to no exploration to be found and you don't have much of a choice as to where you should go next.

4) The story is a bit convoluted. It fucks with the Legcay of Kain plot a bit too much. Thankfully, Defiance cleans up almost all of the messes that BO2 made.

It's just not up to LoK standards. The bar set previously by SR1 and SR2 was just too high for BO2 to reach, especially considering that the BO2 team was a completely separate group from the Soul Reaver/Defiance team.

If you pick up Soul Reaver, you'll enjoy the hell out of it. That's the first one I played, and it's the game that got me hooked on the LoK series. SR2 fills in a lot of plot elements that you might be confused about. If you're really interested in the LoK series, here's the order I'd recommend.

Soul Reaver
Soul Reaver 2
Defiance

And then the Blood Omen games if you're interested in filling in the backstory. The Soul Reaver games and Defiance are the most important to the central plot, the Blood Omen games are just basically filler.