KLM is certainly an option, but I would go with United miles if you can, mainly because in my experience, *A has the best US-Europe availability. UA flies IAD-AMS, but there are some other options that aren't bad at all - UA and SN both fly IAD-BRU (short train/flight to AMS, include flight in award if you want), or do a 1-stop with any of the tons of *A flights IAD-Europe (LH to FRA/MUC, SK to CPH, UA to LHR/CDG/tons more). Remember that SN award flights don't appear on united.com, so use ANA to get an idea of how their availability is.

Sounds like you've got a lot of the major airline cards already. May be time to look into biz cards - Ink and United business, both currently 50K sign up bonuses. Did you get CSP and/or United > 24 months ago? If so, you can cancel and be eligible for bonus again. If you can get a solid offer for Amex PRG (50K is best I've seen), it'd be worthwhile. Amex points to ANA/Aeroplan could be a good play here, as you will avoid fuel surcharges as long as you fly on UA, SK, SN, TK, and SR, so plenty of options.