I'd say it's a tie between Ease of use and Feature set for me, but if I had to pick one, I'd go with Ease of use - a tool that's easy to learn and use saves me so much time in the long run!
For me, it's a tie between Ease of use and Feature set - if it's easy to use but lacks key features, I'll struggle with it, but if it has all the features but is a pain to use, I'll look for alternatives.
I'd say it's a tie between Ease of use and Feature set for me, but if I had to pick one, Ease of use would win - a steep learning curve can be a major productivity killer!