It's been a while since I browsed through the free themes, but I use Effector for my webcomic's tumblr :
https://www.tumblr.com/theme/17403
I like it a lot because it has a lot of easy options to customise and make it look nice without having to dig into the code -- you can set the sidebar on the left or right, or hide it altogether, you can set the content width of posts to 500, 600, or 700 pixels depending on what makes your art look best (I really don't like it when blogs stretch the art so it's too huge for the screen), there's a spot in the basic options to replace the blog title with a custom header image so people can see your art right away, etc. etc.
(my personal tumblr on the other hand is a mess atm so I'm probably gonna watch this thread for good suggestions too xD )