Unfortunately the Tiger Admin plugin doesn't work with Wordpress 2.1 and above. I use Spotmilk to theme my admin though.
The best Wordpress plugin has to be the
Bad Behavior spam filter. You never take spam for granted until after you turn this plugin off. The Akismet spam filter may be good, but this blocks spammers from actually accessing your blog to begin with.
Then again, I use a shit load of Wordpress plugins on my blog to begin with so I've got no real preference. The complete list with the links can be found
here.