Plot Twist:
Best:
- Vampire: The Masquerade
- Arcanum

Worst:
- Any *ahem* "Plot twist" from any Final Fantasy game.

Fight:
Best:
- Last BG 2 fight.
- Last Vampire: TM Redemption fight.

Worst:
- Last NOX fight.

City:
Best:
- Athkatla from BG II.
- Tarant from Arcanum.

Worst:
- Any "city" in Diablo 2.
- Any "city" in Final Fantasy (any number).

NPC:
Best:
- Dog (from Fallout)
- Dog (from Arcanum)
- Jaheira (from BG2)
- Minsc (from BG2)

Worst:
- Imoen a.k.a. Shuddupgirl from BG 1 and BG2 (she's worse in BG2)

Weapon:
Best:
- Mechanical Dagger from Arcanum
- Shock Rifle from Fallout 2

Worst:
- The "Uber" weapons of Final Fantasy.
Not only they look like a big joke but, guess what, they ARE a big joke!
- Stillwater Blade in Arcanum (WAAAY too powerful).

Spell:
Best:
- Ferral Claws from V:TM Redemption
- Wish from BG 2

Worst:
- 3/4 of Arcanum's Water magick

Skill:
Best:
- Backstab in Arcanum.
- Persuation in Arcanum.
- Outdoorsman in Fallout.

Worst:
- Heal in Arcanum

Quest or sidequest:
Best:
- Iron Clan sidequest in Arcanum.
- Dwarven Family Heirloom in Arcanum
- 80% of Arcanum's quests and sidequests.

Worst:
- Diablo 1 and 2 as a whole.

Hardest quest:
- BG 2 red dragon on highest difficulty.
I don't remember the rest of the quests.

Hardest game to beat:
- A bug-ridden one.