I've played all classes to 50+ and the absolute best soloing AC is a Scoundrel/Operative, nothing touches them, its not even close..
What makes them so good is stealth, heals, the best cc of all classes, vanish/combat reset
With those tools you can solo almost all HC4 and the majority of FP's at minimum lvl requirement. A well geared Shadow/Sin in tank spec with healer comp will find a lot of stuff impossible. Commando/Merc and Sage/Sorc in heal spec could solo a lot of HC4 and FP's but without stealth that means trash clearing which takes time, and in most cases with the limited cc both have, makes the trash harder than the actual Champion/Boss.
MMO's are meant to be played with other people but there definitely is a fun challenging element to soloing stuff in game which is designed for group play, and if you like to solo stuff in Swtor then the best equipped AC for doing that is a Scoundrel or Operative (and its not up for debate)