2x2: apparently the newer WitTwo v1s aren't as good as the older ones, but the DaYan is good. That's what I've heard.
3x3: the ZhanChi, WeiLong, ShuangRen v2 and AoLong are all good. It all depends on what you prefer. The AoLong is more controllable than the WeiLong, but is still reasonably fast. Mine isn't as fast as my ShuangRen, but my ShuangRen is very broken in, so the AoLong would probably be a better choice. If your current main is a ZhanChi, then maybe you could get another one; it's still a very good cube, and you are used to it. If I were you, I'd get both an AoLong and a ZhanChi and choose whichever you prefer, but as I said, without knowing which types of cube you prefer it will be hard to choose just one cube.
4x4: idk about anything other than ShengShous, but I've heard the AoSu is best, so if you can afford it then get one. The ShengShou isn't as good, but is much cheaper.