Pay attention to who maps things. Whenever you find something you like, take note of who mapped it, maybe even check out their other maps. Each mapper tends to develop their own style, so when you find a mapper who's style you like, you're more likely to enjoy their other maps too.
Also, give completely random maps a chance. When checking pending/wip maps I usually check to see what the hardest difficulty is. If a map has one difficulty and it's an Easy, I won't even look at it because I don't play easy stuff, but if a map has 1 Insane diff, I'll check it because I know if it's good I'll probably keep it.
You don't need to use multi to find good unranked maps, but it can help if you find a game where people are playing good unranked sutff. Multi can also help you find decent unsubmitted stuff if you're lucky.