Best X Factors to use?

Mainly asking for defensive players, but which X factors do yall use and which positions do you put them on? 

For me I've always done my user LB, safety and corner. Looking to experiment with different x factors. Seeing if anybody else uses the Selfless one with a reduced objective ability stack to get 2 players in the zone? Never done it before. Anybody ever tried it? Id have Zone Hawk and Universal Coverage activated for my corner and safety.