Rogue and warrior are the toughest, but then you're also left with a really shitty character that can't do much besides painstakingly kill low blues in areas where these can be single-pulled. It's the hardest SSF challenge, but one that feels kinda hollow because there's no meaningful end product, you just do it purely for the achievement. It's not as if an SSF rogue will one day become capable of doing something really impressive. The character will be total trash forever.
So in terms of having some future potential, the knights have the best balance between that and challenge. Shadowknights can fear kite their way through life, but can't do a whole lot in a dungeon, so that feels more like a one-trick pony. Paladin, however, can do a lot of shit and has a number of worthwhile things to pursue as you can solo stuff like Ghoulbane, most (all?) of the Ro set, and potentially even make a Hail Mary attempt at camping an FBSS once you're in the upper 50s.
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