Join Date: Jul 2002
Location: Still going through ORD security when boarding has already started
Programs: AA EXP, UA Silver, HH Gold, Bonvoy Gold, IHG Plat, Hyatt Disco, Reno Air MEGA Platinum
Posts: 10,289
Must be UA if you're flying ORD-PEK nonstop.
If that's true, your only option will be one of the 4 United Red Carpet Club lounge. There is a newer lounge near B18 as well as the "main" club near C16.
The other two are not worth visiting.
Honestly, I would just pick whichever one is closest to your gate.