ORIGINAL: Retaks
I picked 22-250 but a 7mm-08 would be great. It would fit right in with the rifles you already have and makes a great long range varmint round or deer round with the proper choice of bullets. The 25-06 is also a great choice for both deer and varmints with proper bullet choices.
I don't think a 7mm-08 enhances his collection at all. There's nothing it can do that you can't do just as well with the two rifles he's already got.
I vote for the .223. In an AR platform.... while you still can.