It's all a matter of opinion.
My opinion is Tweetie and Birdfeed are the only two Twitter clients worth having. Both are very well done in terms of user interface. The others are too cluttered and stuff is badly designed. Read the blog entry about Birdfeed's geolocation support. Honestly, it's brilliant. It's these types of things you DON'T see in those "support everything" apps like Twittelator and such. Birdfeed's developer and designer take their time figuring out how best to do something. It may take them longer but in the end the design is much better.
For me it isn't about the features per say. It's about how seamlessly the features are integrated and how much sense it makes when you use them. I hate hunting for how to use a feature. Just because you can think of a neat way of making a feature also doesn't mean you should cram it into your app. Simplicity is fantastic and too many features clutter your UI and make users scream.
Birdfeed is probably the only twitter app i've used that is actually EASY to read. you can easily distinguish between your tweets, your friends tweets, mentions, etc. The colors aren't obnoxious, the spacing is fantastic, and the font size is perfect. Attention to detail is what matters for me. I'd shoot myself in the skull after using ugly apps like Twittelator for a week.
Again, that's my OPINION. You're welcome to have your own. But realize that what works for you doesn't mean it will work for someone else. You may not have the same priorities in a twitter client as me. That's perfectly acceptable. Variety is a good thing and so is competition.
