I played Agent on launch, went Sniper, once you get SCORPIO you can do anything, even some elite areas completely solo. The companion story lines were also very engaging and interesting, so I would suggest going down that rabbit hole.
The main reason I quit back in the day is due to the nurf to level 40 dungeon credit drops. Could solo clear in a few minutes and rack up shit tons of credits, meaning I could buy the best crystals with ease. Well, that and the completely fucking broken mechanics of the heroic raids pissing off my entire guild, then having nothing to fucking do after. We mostly had all of our booster codex things (items that gave a permanent +1 stat, including the rainbow one +1 to all stats that was insane to get to), so after wiping to the rancor due to shifty LOS issues, or getting obliterated by the shitty laser robots on the way to the next boss... Oh right and I also went grey before they made that a thing, which meant I lost out on the benefits of being light or dark side.
I played through Sniper, Jedi Guardian (?), Sith Assassin and Bounty Hunter quest lines. Imperial Agent was by far the best, with Assassin close behind. The Jedi quest line sucked donkey dick, especially if you went dark, where everyone's like "you're kind of an asshole, but could you help us do X?"...no, fuck off, lol.