I always recommend Imgur because dropbox is blocked at an increasing number of workplaces and it sucks to not be able to see all of the inspirational work.
They just unblocked dropbox here at work oddly enough, but before that google drive seemed to do the job, as well as imgur. i find some people use photobucket as well
Imgur is probably the best at the moment, large sizes, no compression, doesn't over spam ads. Also they tend to keep images hosted on the site a lot longer than imageshack and other sites, and they don't take stuff down if it gets too much traffic (like dropbox).