Best one I ever played was Forgotten Hope mod for BF1942.

rtcw

Red Orchestra 2 would be my pick.

Call of Duty 1

You're not going to get many/any answers for "WW1 multiplayer" games. There are only a few that aren't in the hardcore grand strategy style.

Call of Duty 2 is still the best! :)

cod world at war, company of heroes

MOHAA

Verdun

WWI - Battlefield 1918 mod for BF1942.(haven't played many WWI things.) WWII - Red orchestra, Red orchestra 2, and Forgotten hope 1 and 2.

MoHAA & Call of Duty: United Offensive

Skyrim.