Sucker Punch is the most useful STAB option, as it allows Mega Absol to KO would-be revenge killers such as Talonflame. On the other hand, Pursuit can be useful because of the number of switches that Mega Absol will naturally cause. It works especially well on weakened Pokemon that might want to escape from Mega Absol, such as Talonflame...
...it gets priority, meaning that certain Pokemon, namely Talonflame and Swift Swim sweepers can't reliably revenge kill it.