Taral V -> Maelstrom Prison -> Esseles -> False Emperor -> Battle for Ilum -> Kaon Under Siege -> Directive 7 -> Lost Island
Edit: You could probably debate the relative positions of False Emperor and Esseles. FE has rougher trash, but the 'tough' bosses boil down to gimmicks. Ironfist can be genuinely difficult on Esseles, but the rest is pretty easy.
Thank you very much