Sarevok has also the most detailed character and agenda out of the three, and both of these are executed in a more subtle manner than Irenicus` or Melissan`s. We only get to see bits and pieces of information about what Sarevok is trying to accomplish and at the beginning, we do not even know who he is, we only see the results and consequences of his actions throughout the Sword Coast, and this alone already makes him a more interesting antagonist than the other two. The way his plans are being revealed only bit by bit and then eventually foiled by the player character creates a focused and coherent plot while also providing a driving force for the player and the enjoyment of uncovering a mysterious conspiracy behind the unusal events.
This is only a matter of personal preference, but Sarevok`s story is also a bit more "low-key fantasy" and not as melodramatic as the story of Irenicus, which is also one of the reasons why I like him best. He is a very charismatic and criminal mastermind who also happens to be one of the stronger children of the Lord of Murder and does not hesitate in trying to take up his heritage. However, if the player lets him join the party in ToB, he can also discover a more bitter and down-to-earth side of the character.
Irenicus had a few good lines, but that`s all that he had going for him, and between his hubris and Sarevok`s, the latter`s one has more depth to it.
As for Melissan, well...it was quite obvious from the first time she opened her mouth that she would be the final villain. She was simply an uninteresting and sort of an uninspired character.
Melissan > Sarevok > Irenicus.
As a villain, I agree with all of the above by szablev. The way the game lays out Sarevok is much more pleasing and interesting than in BG2. At first we don't know anything, and then we slowly put the pieces together and then fight him. It's also not really a "Save the world" scenario, which I enjoyed as it breaks with the norm.
Irenicus may be more fleshed out in terms of his motivations, but there is something that's hard to put your finger on that doesn't sit quite right with him. A failed love story makes him want to become a god? Errr. It gives him a more human "face" to be sure, and I actually felt sorry for him, but I thought Sarevok worked better as a villain - not least because of how the game doesn't start off with "SAREVOK is the BIG BAD VILLAIN" from the first second.
Melissan is ridiculously hard to beat, but as a villain there really isn't much to her. It was pretty obvious from the time we met her she was going to be YET another person who would soon backstab us (seriously, how many such NPCs are there in the games? 900?). We do get a little backstory, and that was nice, but there is too little too late to either identify with her or start to detest her.
Sarevok > Irenicus > Melissan
I prefer having a mass murdering brute or something along those lines to fight or even a rogue, that'd be cool.
Im currently replaying Avernum Escape from the Pit and one of the main bad guys is a slith king who is basically just a brutish spear wielding tank. I dont know if there are any other rpgs like bg and avernum that have that, but it would be a cool thing for a change.
I think its cool having mages as bad guys (moritz-kri in avadon was cool), but there has to be more things people can think of. Id really like an Artemis Entreri type bad guy in the next game, that would be fun.
Irenicus was a disappointment. As if he was shoe-horned in as a main villain from a sideplot that went out of control. A pity, since the game setting was at least interesting, if under-developed. It always felt like the creators of BG2 didn't really know anything about the Forgotten Realms campaign setting - just from the start I was installing batteries in some steampunk setting. Never struck me as a true sequel to BG, though a decent enough game on its own merits.
Melissan.. I hardly remember her. Some dumb broad I sliced up, despite the game trying its best to make the fight an unfair one. Utterly forgettable.
Much like the Throne of Bhaal expansion itself.
I would have to disagree about Throne of Bhaal. I think it's the best expansion I've ever played : Watcher's Keep alone is worth buying it for. I'm only acquainted with the Forgotten Realms through the Infinity Engine games, though, so I don't know how faithful it is to the books / pen and paper games. My only major objection against BG2 when it first came out was the disappearance of the wilderness areas, but the excellent stuff it added more than made up for that. I preferred the style of the portaits in BG1 but that's not hugely important. There's not much in it between BG1 and BG2 in my opinion. BG2 has item crafting, dragons, some great spells and many other improvements over the first game, BG1 feels somehow "purer", e.g. Jaheira looks much more like a druid in BG1 (in BG2 she gets a hairdo and makeup) and Aerie is totally annoying, but Jan Jansen is a fantastic creation. I prefer to think of BG1 & 2 as one game and not to choose one over the other, though if forced I would give it to BG2 by the width of a cigarette paper.