UA has recently (two weeks or so I think) withdrawn from DUS and Milan.

UA operates from the US to cities in Europe the following on UA metal:

LHR, CDG, FRA, AMS, BRU, MUC

They code share on many, many more with LH and SAS and others.

Based simply on numbers, the following would probably be easiest to redeem flights:

LHR (13 Flights a Day)
FRA (5 Flights a Day)
CDG (3 Flights a Day)
AMS (2 Flights a Day)
MUC (1 Flight a Day)
BRU (2 Flights a day, one from LHR though, tag)