I've been using K2 on my blog and it's been working fine so far. K2 is easy to customize and there have been no problems with adding new plugins.
Plugins I recommend using:
AdSense Manager
Akismet (included in WP download)
All in One
SEO Pack
Feedburner Feed Replacement
Get Recent Comments
RSS Footer
Subscribe To Comments
WordPress Database Backup
Also useful if you need them:
CForms
Increase Sociability
Twitme
Yet Another Related Posts Plugin