Arkham Horror: The Dunwich Horror Expansion – Great expansion, I think it definitely improves the base game. With 4 investigators this is just perfect. Gatebursts and extra locations keep the win from being locked in after sealing a few high probability gates. Injury and Madness cards are awesome too. Can be a little much for teams of 2 or even 3 investigators sometimes though.
Arkham Horror: The King in Yellow Expansion – I feel like the Acts are too hard if you play as recommended, and too easy if you just shuffle them into the deck. Otherwise, the blight stuff is cool. Also, the item decks are getting pretty watered down for those who want weapons. No Dunwich (or other extra board) mythos cards, so, again, this will water down those gates if you add these mythos cards to the expanded game.
Bang! A Fistful of Cards – Nice card additions spice up the base game, but sadly they do nothing to fix the pointlessness of the system. Recommended if only because it at least puts a time limit on the proceedings.
Dungeon Twister: 3/4 Player Expansion – If you find yourself in the rare situation of being in a game group with more than one other Dungeon Twister fanatic, this is great! Though, even at 3, downtime becomes an issue, and adding extra players does nothing for your ability to control anything beyond the tactical level.
Lord of the Rings: Friends & Foes – Definitely improves the base game by making it a bit harder, and giving lots more options. However, due to how the boards work out, I only ever really see people taking one route through this (ie, ALWAYS skip the two boards you can skip), so that feels a little gamey.
Ticket to Ride: Netherlands – The toll mechanic really does provide a nice incentive to play aggressively. The scores are unusually high this time, but it’s still one of the better expansions.