I personally like the Blue Zinfandel Enhanced theme. It is easy to customize and can be manipulated to fit a lot of uses.
I don't think there's one single best WordPress theme, hands down. Really all depends on what you're looking for. Do you want a magazine theme, 3 column, 2 column, sidebar location? There are TONS of good free ones out there, but I especially like the ones Smashing Magazine features. 20 More Free First-Class Wordpress Themes is their most recent. There's a few more lists on SmashingMagazine too...good luck!
We all know the default wordpress theme would be the obvious! This is a very common theme that I see almost everywhere: Credit: http://www.hongkiat.com/blog/49-most-downloaded-wordpress-themes-of-all-time/
The Ads theme is real easy to customize: http://www.blogging-secret.com/wordpress-themes-free-download
This list should start you off: www(dot)smashingmagazine(dot)com/2008/01/08/100-excellent-free-high-quality-wordpress-themes/ - err sorry can't post links yet. Didn't want to SPAM other boards so err..
I like the I feel dirty theme. I use it on my blog, this is how it looks like: http://tech-talking.com/
I was asking for opinions, not lists or websites for them. Yeah, that's a pretty cool looking one. Gigacore, way to spam your own links...and we aren't looking for theme websites, just the best theme.
http://courtneytuttle.com/seo-wordpress-themes/ has free templates that are seo-optimized and checked for working in different browsers, I am using http://courtneytuttle.com/seo-wordpress-themes/cutline-3-column-split-11/ and like it very much. SY
Mimbo looks good, I think the CSS was kind of messy on that one though (unless I'm confusing it with Morning After)
Best free theme should firstly be a magazine theme. Regular blog themes are almost outta fashion. Anyway, if you ask me, I'll say the competition is between, The Morning After, Mimbo, Small Potato Magazine Theme, Silhouette, Dezinerfolio D2 Theme, and Wp-Polaroid. They are simply mind blowing and in my opinion no other themes can beat em.