I'm thinking singer/lead guitarist, like Kurt Cobain and Eric Clapton have the hardest job in the band. Guys that sing and play bass have a tough job too, but bass tends to be more repetitive than lead guitar so you can focus more on singing. Same for singer/drummers like Phil Collins. Although I will say that most singer/lead guitarists, tend to shift their focus back and forth. Meaning if there's a hard part to sing, they'll either stop playing the guitar for that section, or they'll play a very easy repetitve pattern. And then when it comes time for a difficult guitar part, usually the solo, they'll stop singing. I mean, duh, right? That's what makes it a guitar solo. But what if it's only that way because we've been conditioned to expect it to be that way? Maybe there could be a section of a song that's as complex as a guitar solo, but where the dude sings over it anyway... if there was someone talented enough to pull that off.