Geez, it'd be a toss-up for me between Legend of Grimrock or Trine.
I've never been big on point-and-click adventure games, so Machinarium, Botanicula, and The Whispered World are out. And I haven't played Darwinia, but I can't say it looks to be something I'd easily get into either.
Between Trine and Grimrock, I guess it depends on what you're more interested in trying out: a platformer with 3 hot-swappable characters with different characteristics and helpful techniques, or a first-person dungeon crawler like Eye of the Beholder (but much prettier).
Trine, like I said, has you controlling one of 3 characters at any given time: a warrior with a shield to deflect attacks and push objects, a wizard that can create boxes/planks to help you maneuver or take out enemies, or a rogue with a grappling hook type of item to help you swing from place to place and a bow to shoot enemies from afar.
Unfortunately, I can't say much about Grimrock since it isn't released as of yet, but going from a review by TotalBiscuit (YouTube "WTF is: Legend of Grimrock" for his first impressions) it looks to be a very pretty dungeon-crawler in the style of EotB and the like. It has real-time fighting, decent inventory system, good character/enemy design, puzzles, etc.
If I had to choose, I'd pick Grimrock if only because I've had Trine for awhile now and Grimrock would be a new experience.