The LH Sen lounge B at FRA is quite nice IMO. So is the Sen Lounge at MUC (G24), and it's even used as an intermittent F lounge as the G-gates FCL is under renovation.
I'll also miss the United Club at LHR, I haven't tried any polaris, but I've heard good stuff about UA Club LHR and had the same thoughts during my visits.
Haven't really tried any SkyTeam as I've shifted everything over to LH, but the KL Crown Lounge was ridiculously crowded that one time I was there.
I guess you haven't been there for a while. They broke down the lounge and opened up the small rooms into a big space. Removed all the nice sofa couches and lounge chairs... Only thing still the same is the entry/lobby.
I was there in April, and there were still separate rooms?