For coms you definitely want to max your buffs: increase mass, kinetic reservoir, and growing rage.

For pvp:

one of the combos that i like to use is singularity-->stomp-->backbreaker
this combo usually guarantee a knockdown and hopefully a combo which is better than having only one knockdown skill.

For pve:

Usually you would stay away from the skills that are single targeted based. nebula is a good skill to use while farming in pve