I've had Android for less than a year and I gotta admit, the only thing I miss is Gravity for Symbian.
I use the official twitter client, mostly because I don't see that much of a difference from the 3rd party bunch.
The 3 features I miss most are:

1- Image browsing for a list, the timeline or an user.
2- Image resizing before uploading.
3- Having lists and searches as a column, like the timeline and mentions.

If I had this 3, I'd be really happy