MSN Talk is the best I've tried, and I have tried several apps.
It is highly customisable (background, text and shapes colours, contact list layout and style etc.) and very stable. I find it to be the most responsive application since it displays quickly people's avatar and status changes etc.
Here are 10 features I consider major pros in no particular order:
1. Reconnects when disconnected (optinal).
2. Option to add your own avatar whilst it displays other people's.
3. Decent notifications for messages, nudges etc. and fully customisable.
4. All status options available (i.e. Available, Busy, Away, Appear Offline).
5. Shows complete personal messages of your contacts.
6. Easy to retrieve a single conversation or revert to the main contact list.
7. It supports nudges and all standard emoticons (both send and receive).
8. Sign out, quit or minimise options.
9. Option to switch off wi-fi after a certain inactivity time.
10. Any setting you can possibly think of for a mobile messenger application.
I hope this helps.
