apc vs eaccelerator vs xcache
APC is going to be included in PHP 6, and I'd guess it has been chosen for good reason :)
It's fairly easy to install and certainly speeds things up.
APC definitely. It's written by the PHP guys, so even though it might not share the highest speeds, you can bet on the fact it's the highest quality.
Plus you get some other nifty features I use all the time (http://www.php.net/apc).