I'm using Nulab's Backlog. This is sort of a shameless plug since I work for that company. However I must say that after several years of clunky issue trackers, this one feels very very refreshing. I won't waste your time with marketing speech, which you can find plenty of on the official website. But actually easiness of use and moderate configurability, plus a decent-looking UI, are the points our advocates praise most often.

The company is very strong in Japan, where it was founded, but rather low-key in the rest of the world. I never heard of it myself before I joined, but after starting to actually use the product, I really wish I had.