the easiest and fastest solo clases are scoundrel/operative
there are a few reasons for this
1 - companion healers are not as good as companion tanks. the healers will have a hard time keeping up a player tank vs. a champion as you level if this is not an overgeared alt. on the other hand a healing hybrid spec is viable for sage/sorc/merc/commando and companion tanks are very, very tough.
2 - cc. being able to stealth your way around and pick your fight along with being able to knock something out of comission provided it isn't a droid (you can then knock the droid out after combat starts) is incredibly handy
and lastly, of the hybrid healing setups the scoundrel/operative do it easier while maintaining resources. oftentimes just keeping a 2 stack of the hot rolling is all your tank companion needs while you destroy that champion from the rear. your tank companion will eat any aoe stuns and cc from the champion, leaving you free to cleanse and heal him up.
sadly come end game neither class holds a candle dps wise anymore thanks to heavy handed nerfing of their burst but no compensation in overall dps, and are generally relegated to healbotting to get anywhere.
I COMPLETELY disagree with #1. My first character was a JK guardian tank (full defense spec) and when I got my healer companion (Doc) I became invincible. Yes, fights (and by extension quests) took longer to complete, but I had no fear of dying and never had to stop to regen my health. And due to low sever population, I was all but forced to solo that character to 50. Furthermore, on my mercenary Bounty Hunter (where the first companion is a healer), I specced DPS, and so long as I did not run head long into packs of strongs and above (using my own and my companion's CC), I was again invincible.