I guess it depends on your definition of "hard". Controllers are squishy and do crap for damage, but their controls make soloing relatively easy. Slow, but easy.
Defenders are also squishy, do a bit more damage than Controllers, but their primaries usually make up for it. Again, slow, but easy enough.
I'm gonna say a Peacebringer. Middling damage, minimal buffs/debuffs, no mez protection till Light Form, and not-so-great resistances. Also a massive end hog. If you want to make it an extra-painful experience, do a human-only PB.
Don't get me wrong, I love Kheldians. but they simply aren't all that solo-friendly.