This is great. It's always a pain in the neck to open up an editor to do something as small as resize to make an avatar. Another thing I like about this is that the image is already in your browser and to-size, so you can kind of get a feel for how it's going to look on an actual page.
I don't know if it's the best, but it's the only one I use. It's simple interface is what I find most useful about it. If I need more (which is rare), I use the GIMP.