If one is looking for sheer volume, Best Western has 1,312 locations in Europe. I believe (but not sure) that BW has more affiliates in Europe than any other group.

If find IHG to be pretty omnipresent in my European travels. If I were a biz traveler within Europe and paid for it with my own dime I would probably choose IHG. But since I travel within the U.S. and my European vacations are to major cities, Hilton has proved very good for me as I can vaca in a very luxurious hotel for points. I typically earn with Hampton in the U.S. and then redeem in Europe (and Hawaii) at Hilton hotels.

Last year Hilton devalued their HHonors program so when my Diamond status with them runs out this year I will most likely switch to IHG properties in the U.S. I will still get to earn through modest properties (HI Express, HI, Candelwood, and Staybridge) and redeem in Europe at Intercontinental hotels.