1. Google Reader - This is my top webapp. Since the iPhone doesn't have a built-in RSS reader, and I didn't want to jailbreak mine just to install one, I use Google's web-based RSS reader for my daily news fix. They've optimized the display just for the iPhone so it's very easy to use.
2. Digg - My day isn't complete without reading up on one of the best news sites on the internet. Although the news isn't exactly the most relevant, they've divided the news articles by Topics which make it easier to read through. Props to Kevin and team!
3. Facebook - What can I say? I'm extremely attached to my Facebook, and it's iPhone webapp just shows me the right amount of information with a great, easy to navigate interface. I only wish though that it also supports Facebook applications. How about it Superpoke?
4. Flickr - I don't have a lot of pictures in Flickr, but if I did, I'd fully recommend using the iPhone interface. It shows interesting photos on the top screen, with a search bar at the bottom for searching through public photos. Once you're logged in, it shows your photos in a 3x2 grid, and clicking on the photo zooms in. The page then shows the image with navigation arrows for the next and previous photo. Very simple and slick!
5. Twitter - Admittedly, I'm not a Twitter addict yet. However, with a very simple interface that gets right to the point, I give 5 stars to the Twitter iPhone webapp. The homepage just shows the ever familiar What are you doing? field, and a list of posts by people you follow. What's else do you need?
And that's it folks. What's your favorite webapp? I'd like to hear your comments!