I personally liked MGS2, it was fun to play, looked good and provided a lot of hours of gameplay. Now, not having really thought about the game in the context of comparing it to the others, I was planning on defending it, and to a degree, I will.
The game told a great story, Raiden was intended not to really be liked, because he's not Snake. He's the opposite of Snake, more effeminate, he’s “green” and not a pro like Snake. That is the point, Hideo gave you a character you would hate, so he could tell a story and make you like him. He really is a badass, and is tied into the whole “Snake” family. The other big thing to remember here, in regards to the MGS story is that in MGS2, everyone didn’t want to be Raiden and by the time MGS4 came out everyone was mad you COULDN’T be Raiden. Kojima did exactly what he had intended to do, give you something he knew you would hate, and make you love it.
Now, onto Solidus; While not my favorite Snake, we need to acknowledge that he wasn’t just a Deus ex Machina thrown in to provide more story, as it is mentioned in MSG1 that there was three brothers. And by knowing this, he set up the perfect next enemy for the next installment.
1Next is the bosses, I disagree that they are the worst of the series, and the fact that you can't kill them adds suspense and makes the return of that villian more engaging. The fight with Fatman, and trying to find the bomb he is sitting on, (Who you kill if I am not mistaken as Raiden) my personal favorite, the fight with Fortune, where you literally can't shoot her, and even when you know this now, play it again and tell me you won't shoot at her anyway, just because it feels like you need to, you're helpless, but to hell if your not going to fight it (and the similarly with Vamp). Now, I personally think that the villians in MGS2 are the most believable. They explain all of the reasons for the superhuman powers that the Dead Cell (also, Dead Cell is by far the coolest group name in the MGS series) members have. Unlike MGS3 and MGS1 in which some people are just magic.
That said, don't get me wrong, I like Psycho Mantis just being psychic as well as the other powers of those in MGS3. But in comparison, we have a man who communicate with the dead, a russian general who constantly has 10 million volts running through him and a super bee tamer. In comparison, MGS2 has the most reasonable characters. And their lack of magical powers (spoilers, its electromagnetism and nanomachines)
2,3 makes their realization of this (specifically Fortune stopping MG Ray's missile.) that much more powerful.
MGS2 is by no means perfect, nor my favorite. The story gets convoluted FAST and the relationship between the La le lou le lo, the Patriots and so on takes sparknotes, a whiteboard and a few days to really digest. Also, the overly powerful symbolism, (the fight on top of the Federal building specifically) gets a little to hard to swallow, as well as it being, in my opinion, the "talkiest" of the MGS games (forgiving MGS4 due to the fact it had a lot of loose ends to tie up and for MGS stories, that takes awhile)
1 http://en.wikipedia.org/wiki/List_of_characters_in_the_Metal_Gear_series#Solidus_Snake2 http://en.wikipedia.org/wiki/List_of_characters_in_the_Metal_Gear_series#Fortune3 http://en.wikipedia.org/wiki/List_of_characters_in_the_Metal_Gear_series#Vamp