I think if you mean just
as
a character, at all, Scarlet Witch wins because she gets carried hard by the MCU, but obviously I don't think most people who recognize her now recognize her as an X-Men character
Mystique, Nightcrawler, Colossus and (not on the poll) Quicksilver also seem like good picks because there were pretty prominent in movies and are otherwise very memorable
I love Gambit but I think him not really being in much notable media since the 90's really hurts him. I actually do remember the arguments from the MvC3 board where people wanting Gambit and Venom in MvC3 would get in huge arguments with people over them being "stuck in the 90's".
Born to lose, live to win!