Judging from what we're given from the show, who would win in a fight?

Lord Tirek, or King Sombra?

If Tirek's got sufficient power already absorbed, he'll be able to just drain Smokebra dry. If not, Sombra just locks him in a nightmare and impales him with a crystal.

Really, it all depends on how well Tirek's doing, his strength is very variable.