I'm really partial to a lot of the European metrics. My current list reads something like this:
8x68S - Wanted to build this in gunsmithing school but was pressed for time so I had to go with Plan B. I have all of the components and even bought the reamer and head-space gauges so someday this will get built. (Had an instructor load up some ammo too.)
9.5x66 Vom Hofe - A super-powerful Euro .375 that can fit in a standard action. It's a big thumper that's non-belted - what's not to like?
11.2x72 Schuler - Everybody needs a Dangerous Game rifle at some point in their lives. This is my choice for that.