Sfo
I've only been to HKG, SFO, IAD, LHR and ORD RCCs, so I don't have great comparisons. As far as I am concerned if a lounge doesn't have a shower, expects you to pay for drinks (tips are virtually required even with the 2 free drinks with a full fare C class ticket) and has essentially no food beyond salty snacks, it's dire.
UA may redeem itself with its newly announced C class seat, but its overall product still remains well below what NZ, QF, SQ, CX, BA, VS and LH provide, among others.
I already use the SQ lounge at LHR whenever I fly NZ (or UA) from Terminal 3, but it will be interesting to see what that and the UA lounges are used for when Terminal 3 becomes OneWorld + VS terminal after Terminal 5 opens.