I feel UA Mileage Plus is best - its easy to get mileage upgrades and awards within USA if you are vigilant and use something like KVS tool. Had 13 flights with United last week - all my UDUs came through (4 out of 4), they recognize your elite status and the staff are great, despite what else you may read here. The other major advantage is that its dead easy to book mileage awards through the UA website with Lufthansa using your United miles, with low low low taxes. Fancy a trip to TLV in C or F ex MUC or FRA? Tryout United
If your not flying so many miles and would like Star Gold then A3 is great - their network is good within EU, their lounges are fantastic, and the Star Gold lounge access and additional baggage are available for little cost in terms of flying if you choose your fares well
LH Senator is great - they block seats next to you (IME with 20 flights with LH the past summer travelling in Y and C), allow access to exit rows at checkin and generally superior service onboard