I've been gone from this game for quite some time now and just returned recently and a LOT has changed. I decided to start over new and moved onto a different server but now I seem to have Problems with finding the class I like.
etc. etc. etc.
well, let's see. i'd say any class is viable for end game pve. all it takes is practice and knowledge of ops. any class can level to 50 easily too.
i have 5 level 50s so far (guardian, shadow, gunslinger, scoundrel, powertech). i enjoyed them all, but my favorite would be my two smugglers. my gunslinger is my main. really love the cover system. it takes a little while to get used to it, but it's lots of fun when you get the hand of it. scoundrel was an enjoyable class too. they are more like rogues with heals. less dependent on the cover system, so you could always go that route if you don't want to deal with cover. scoundrels also make good healers. more mobile than sages, and they get two 60 second cc skills.
my guardian is my main tank. honestly, i don't think i've had as much fun tanking in other games as i do when playing my guardian. i know you didn't sound like you wanted to tank, but it's a challenge and feels great when you can control fights. shadow tanking is much easier than guardian tanking if you feel like giving it another go, not to mention they get stealth.