To be honest, out of the 2 I prefer the first one, because it had lots of open space/forest areas, something I just missed in BG2. And because you played at lower levels with weaker items, so you really learnt to appreciate that Long Sword +2 or 5th level spell (ToB was a Monty Haul towards the end. "So I found this elven chainmail +4 + 80 vs undead. I'll just toss it with the 20 others I already have."). And probably for nostalgic reasons.
But yes, the NPC interactions, extra kits, class quests and stronghold... etc were way better in BG2.

Morrowind would be for me, I don't claim to know the game blindfolded but I know it fairly well and would spend hundreds of hours learning it all over again.

The Second Sky, no contest, even though I hadn't played the title hold yet. I just wish I hadn't been logged into the chat on my first day:

"The fact that [REDACTED] exists in the world is by itself a huge spoiler for TSS.
"I hope I didn't spoil it for anyone in the chat."

ಠ_ಠ

Diablo 2

Would have to be a tie between Civilizaton III and Civilization: Test of Time. I'd like to forget them and replay either of them to give them a more fair playthrough. I was a bit too harsh on them the first time i played the games because at the time Civ II was my favourite game.

Civilization III had a lot of flaws when it first came out that was fixed with the expansions and made it a much, much better game. Think i'd have really liked that one if i had given it a second chance. (to this date it is my least played Civ game)

With Civilization II: Test of Time i was put off by the new art/animations of the game, compared to the original, to enjoy the gameplay. That's probably my biggest complaint of that game really. :P A complaint i could look away from now if i replayed it again.

Hmm, forget all the past experience you had with the game and play it as if it's new to you? There's an easy pick: Gothic 3.
I get to forget all the bugs, messed up textures and freezes and play a fully patched game that it is now. Yeah, sounds good to me.

So difficult to choose only one... I would say all the Infinity Engine cRPGs plus Neverwinter Nights but as I'm supposed to choose only one, that would be Baldur's Gate II.

Why would I want to forget a game?

If I had a game-related superpower, I'd want it to be the ability to restore PlanetElderScrolls to life. Or for me to get a copy of everything that was there. So many mods with broken links now.

Silent Hill 2

I don't find the idea of completely forgetting a game an playing it all over again that appealing but if I had to choose one it would be either Planescape Torment or Morrowind.