For Mac I prefer Echofon. I tried the official Twitter mac client, but didn't care for it. For iOS I think Tweetbot is the best out, but Echofon is decent as well, it just lacks the sleek UI that Tweetbot has. Then for Android I always liked Tweetcaster (pro) best. It just looked better to me, the layout was better, performed better, and had the most features that I actually cared about. Plume was decent, but always took ages to refresh.