Well in single player Lightning for mass destruction of stormtroopers, heal for some lame immortality (you can always run and spamm heal), absorbtion for protection against *sometimes* annoying force powers and grip for funnier gameplay.
Multiplayer: Uh.. that depends. I never played a lot of siege/ctf/ffa, I more or less concentrated on Duels without force.
Anyway, in duels with force (unmodded) drain is lame. heal yourself while draining the force of your opponent...
Drain counters everything. Absorbtion protects against it, but that's not an offensive power. If you skill drain, push and force sight, you are virtually not beatable through force powers.
FFA is another story...that lame modded grip kick.