Well I've only done three myself so I'm not the most experienced Track Dayer on this site by a long shot.
I've done one with this club which had No Limits track days involved helping us out. I've done one with No Limits and one with Focussed Events and they were all good. Our very own WeeJohnnyB is an instructor with No Limits.
Apex Trackdays are also supposed to be pretty good.