I'm personally partial to Twellman, personally. Lalas is basically openly a troll, which I tolerate more for the US than I would for, say, English football, because it's ok if we have one dedicated troll.
Holden has improved a lot over the past year, and I'm glad for him, because he seemed awfully nervous for a while.
I think Clint Irwin (GK for the Rapids) might be good in the booth someday -- at least, he's very clever on twitter, which may or may not translate.