On iOS, there's just so many good ones that I find myself switching it up on a weekly or bi-weekly basis. Tweetbot, Twitterriffic, Tweetlogix, Ubersocial Pro... I like what people refer to as the official Twitter client, but it is in dire need of a dark theme. And for some reason, it doesn't display inline images on the iPad like it does on the iPhone. If that makes sense to anyone, please let me know.
And as good as Tweetbot is, the sounds SUCK. They just plain SUCK. It's worse than fingernails on a chalkboard. That's why I think I give the nod to the best overall client to Twitterriffic.