I found over and over again, playing maps with songs that I enjoy, but are too hard for me, are always good for improvement. I strongly disagree with people saying "playing hard maps to fail over and over again won't help you." Playing maps that are nowhere near what you can do won't help you, and there's truth in that, but playing maps that are hard, and ones that you don't fail 10 seconds in, are very beneficial.
I used to fail airman consistently within the first minute or so, then I randomly played it, and did pretty well (almost at the very end). A day later, I played it again, and I passed it. It was a map I felt that I didn't think I could pass, but wasn't one that I felt like I have no hope for (like Big Black or Freedom Dive).
I think the best maps to improve would be for you to map them yourself. You know what you like, you know what you dislike, you know what you're good at, you know what you suck at. Mix them up, or make beatmaps that have only those things you're good/bad/like/dislike.