I'm having a weird sense of deja vu but anyway...
Going by frequency of use:
Tweetie - I've found it to be the fastest Twitter app to quickly check latest tweets and to check on replies, which is what I mainly do on the go.
BeejiveIM - No other IM app comes close, although Palringo is making ground *BUT* I am still waiting to see a revenue model from Palringo as nothing is ever free.
Remember The Milk - This should really be number 1 on my frequency used list but Getting Things Done is yet to fully win out over my procrastination, I'm trying though!
Sounds of Nature - I still prefer the UI of Ambiance but I much prefer the ocean sounds in Sounds of Nature especially the wind chimes.
MyWeather - Good to quickly check the animated radar to see what weather mother nature has in store for us, however they could really improve load times and remembering zoom levels! I really wish Weather Underground came out with a native app for weather, can't believe they did an internet radio app before a weather app!