Extreme Evoboost Eevee is popular, but i'd suggest something that can take a hit/setup well.
Prime choices would be Jirachi, Mew (plus z-transform=extra life, almost), and Celebi. You can also use Pokemon like scolepede and Mr. Mime, but the three mythical Pokemon do a better job because of: rounded stats and access to more stat boosting moves, then again, you can use smergle for any move, but smergle is very frail.
