Dps companion, for all specs.

Unless you're trying to solo MM or something like that. Then you might want to switch to a heal comp with the kolto shell and channeled heal turned off.

They all seem to work fine... what's the best in your opinion?

All three comp roles are good given level sync and the relative power of companions.

Tanks make surprisingly good comps for ops, though. Leveling, tank comps let you get backstabs.

At 75, send your tank comp in at an enemy in a group but at the far end of the group. Then target and click the harpoon ability on the enemy farthest from the first. Your comp will pull three enemies in and then aoe explode right after on all four. This is a perfect set up for your aoe tactical (Viral Elements/Explosive Cells). Aoe mow them all down, then dismiss and resummon your compaion at full health and with full cooldowns. It's a little micromanage-y, but I find it faster on heroics with forced combat than using a dps comp. Especially with groups of all ranged mobs. Groups of golds just melt in the aoe without you having to single target dps them down.