i prefer photobucket.
even though imageshacks came up with username and now, you can save files that you've uploaded, it's not as handy as photobucket.
i've used tinypic before too. but it seems like after sometime, the picture will not be your original pic but of something else, mostly of naked women. this can be quite disturbing if tinypic links are posted at forums. however, it's easy to use. and easy to remember, tinypic.com
the bad thing about photobucket that i really hate is how they'll resize your pic if the size is too big. i once scanned my tutorials and uploaded for a friend, she yelled at me cause she can't read a word.
xs.to is another image uploading site but it's kinda troublesome, in a way you have to checked the terms and condition box everytime you upload something and no thumbnails. (the last time i used it)