Joomla! SEF - I know, it isn't an extension, it's core functionality. But I tend to not use any SEF extensions. The core works fine as far as I am concerned.
Kunena - The forum extension I use on my site. I didn't have a hard time customizing it, which - to me - means it's good :).
Joom!Fish - It has some problems here and there, but it still is the best solution out there for multi-lingual sites.
FLEXIcontent - I haven't had the need to use a CCK yet. But I love the way FLEXIcontent is made and that it uses the core content database tables (means a lot of plugins will not fail to work on it)
ND Editor Switch - It helps me a lot in testing out stuff on the different editors... a time saver.
What? Nothing!- Ok, I lied. This is one of my extensions. But it must be the best extension ever!