I can't play any role that is totally reactive. This means a role that really needs a partner/companion to make things interesting and flow. I am overjoyed when my characters have someone to interact with, but because of my playtimes and the types of concepts I enjoy, it gets tough to both have interaction and be sustainable/survivable.
Examples: A role that is hard to train into a good combatant if their goal is to become one. There are lots of situations that lead to a combative character being unable to advance in combat; examples include being gemmed (lack of access to training once gemmed), not having an IC reason to hunt critters, or having social responsibilities such as examples of concepts I tried before with fighters in merchant house positions.
I love non-combatants that have non-combative abilities; this basically means Merchants. The stealth guilds without a combative focus (pickpocket/burglar) are really hard to enjoy because using those skills stirs up so much shit in game. I don't want to twink-train their skills to hell because that kind of gameplay is just a chore. There aren't too many other role types out there, though. In general crafting is my fall-back for non-combatants; if I have enough crafting to distract myself with the sluggish times are more tolerable, and I can wait and see if the character has hope to make interesting storylines with other players who might intersect with me.
The best combatants have a favorable set of conditions to grow into good fighters; rangers, warriors, and MAYBE assassins (but that requires specific situations/subguilds or allies). The worst combat roles are not adequately positioned geographically/socially to grow and develop. Combative characters who have sub-par stat rolls are also a chore to work with. I enjoy these failed combatants most when they have cool/interesting deaths, but waiting around for death isn't as fun as it sounds, at all. The game is most enjoyable when you want to work hard for something and the uncertainty of whether you will succeed or fail at reaching your character's goals is addicting. Therefore rather than continue to play a non-workable role to completion I often store; of my last 10 characters, I think I stored around 8. The ones I look back fondly on all had a death that meant something to someone or to the setting even in a small way, or lived long enough to see someone else they cared about die before I stored them, satisfied that their life had told a good story.