The absolutely best chair in the world... and seen in many movies, ads, studios, is the Herman Miller Aeron chair.
http://www.hermanmiller.com/aeron/
It's expensive, though. Retails for around $800, and it'll set you back at least $500 on craigslist/ebay. If you've ever sat in one, though, you'd know why. It's just supremely comfortable and you can sit in it for hours without your legs shifting around, butt getting hot, etc.
A step down, but still good, is the Ikea Verksam chair:
http://www.ikea.com/webapp/wcs/stores/servlet/ProductDisplay?storeId=12&langId=- 1&catalogId=10101&productId=43010
It's slightly less expensive, but still one of the more expensive chairs out there. (P.S. I've never owned a Verksam but my friend has one and it's comfortable. I've also read that some people's verksams aren't very good quality so ymmv.)
If you're really not looking to spend more than $100 or so, then just stop by your local Costco or Office Depot. A new chair is better than a lame chair. My experience with these, however, is usually pretty bad. Chairs usually last about 2 years before the seat cushion starts going bad, or parts start falling off, etc.
Be sure to look for strong armrests and firm cushions. Leather is nice but can get hot and sweaty, and my experience with cloth chairs is usually a bit better. Mesh is nice as well (like the Aeron chair). There are a lot of knockoff Aeron chairs out there but none really feel like an Aeron.