Slamming Blood Omen 2 is fandom syndrome. When they tell you it's terrible it's simply because there's something it does as part of their beloved series that they don't like.
The gameplay for Blood Omen 2 is fairly mediocre (especially by the standards of today) and the story is quite straight-forward; (though it gives good coverage to, and fleshes out, a key story element) though I strongly recommend it as a means to get to grips with Kain's character, and appreciate the unique charisma of that unphasable ambition and pride that no other character in popular media matches up to. Any moral self-doubt is quickly put behind him on his inexorable march toward his goal. Kain wants to conquer the world, and nothing will stop him.
I recommend starting with Soul Reaver 2 (which is my favourite of the series). This is because the manual comes with a story segment that backgrounds the history of the world and the series up to that point with detail and a degree of (delightful) romanticization; while trimming out the fat. (most of that coming from the Blood Omen 1 plot)
I think most suggest Soul Reaver 1 because they experienced it first. In my case, I started the series at 2 then went back to 1 afterwards to see the epic things I read about, and found it somewhat disappointing. In my opinion it is much better to read about them in the SR2 archive and enrich the scenes with your imagination than to see them realized in an archaic PS1 game.
Gameplay-wise, SR1 plays very much like (a clunky) SR2 only with blander landscapes, and is more taxing by way of it's opaqueness; as it's an open world with puzzle-based obstacles cleared only by gradually obtained abilities and gives virtually zero guidance on where you should be going or what you should be doing.
Story-wise, SR1 has simplistic cutscenes with a relatively small amount of semi-cryptic dialogue, and then it's back to the gameplay where you'll be figuring out where to go next.
Soul Reaver 2 follows on the PS2, with better and longer cutscenes and dialogue that is easier to follow (but by no means dumbed-down). From SR2 onwards, the characters narrate their adventures more frequently and it helps give more insight into the characters, the plot, and their thoughts as things unfold. The story of Soul Reaver 2 has several turns and twists and Raziel sees more development than in any other game in the series.
The series story archive in the Soul Reaver 2 manual is essential reading (do not skip if you are beginning with SR2). It can also be found and read in the game's bonus content if memory serves correctly, along with the option of watching the opening FMV of Soul Reaver 1. I do not suggest trying to play this game without the help of a manual to go over how the game works, however. There is a main menu cheat code to unlock all of the bonus content for Soul Reaver 2. You should read the story archive and then watch the Soul Reaver 1 opening FMV (which is the best part of Soul Reaver 1) before starting up the game with Soul Reaver 2's equally cool opening FMV.
To give something of a taster/vague overview of what to expect from the stories in the games:
Blood Omen 1 is Kain's initial story and quest. A chain of events leads toward the fate of the world "Nosgoth" hanging in the balance. The old order of Nosgoth is eradicated, and Kain is all that is left standing at this dark hour. Most of the the story and lore from the series is derived from what happens in this game. Read a decently detailed plot guide at least if you don't plan to play the game.
Soul Reaver 1 introduces Raziel. Kain has conquered the world; not without sealing its eternal corruption, of course. The very throne he sits upon is the ruined symbol of Nosgoth's betrayed health. After more than a thousand years, he rules as the undisputed emperor of a wasteland. The plot, while suitably epic, is relatively straight-forward. Having indirectly offended Kain's ego; Raziel, Kain's vampire son, is executed at Kain's command by his own brothers in a heavy-handedly torturous way. Resurrected as a new indestructible and multi-dimensional being by the Elder god of Nosgoth; he's sent out on a mission to kill Kain and his vampiric empire.
Soul Reaver 2 begins as Kain travels into Nosgoth's past, with Raziel in pursuit. Before long, Kain makes a case to Raziel that he had been set-up in a conspiracy to corrupt the world; and that he has returned in effort to find a way to prevent Nosgoth's doomed future. As the story unfolds, Raziel begins to wonder if his quest to kill Kain isn't really as straight-forward as he'd like it to be.
Blood Omen 2. Something of a side story. It covers Kain's quest for world domination not long after the doomsday scenario of Blood Omen 1, in an adjusted timeline. Kain's mighty vampire army suffers a shock defeat to the human's anti-vampire army when a new leader of theirs defeats Kain in battle and takes Kain's all-powerful prized sword the Soul Reaver as his own. Kain awakens 200 years later, thirsty for a rematch over who will ultimately rule Nosgoth.
Defiance picks up where Soul Reaver 2 left off. You switch between Kain and Raziel. In this closing adventure, each character is challenged with the question of whether their attempts to alter the course of history and the grim fates each are faced with, hold any hope. Kain, the bastion of heritage and destiny; and Raziel, the bastion of free-will; are also confronted with fore-tellings of a battle to the death over the fate of Nosgoth.
IMO, Blood Omen 1 and Soul Reaver 1 are stories in which stuff simply happens (interesting stuff, don't get me wrong) and build up to Nosgoth's dystopic wasteland. The SR2 summary is an effective substitute for these experiences IMO.
Soul Reaver 2 onwards chronical the attempts in Nosgoth's past to screw with the timeline in order to prevent the future seen in Soul Reaver. The narrative expands, and Soul Reaver 2 and Blood Omen 2 cover the characterization of Raziel and Kain respectively; while Defiance wraps up the story.
This is my take. I hope you find it useful.
