Brite Touch sandable grey primer. I invested in a good airbrush+compressor and started using gunze 1000, which I love btw. However, it takes a lot of time and cleanup and is a hassle. I've gone through many primers, including Krylon, GW's, Floquil etc. Floquil is good I think, but wasn't working for me and is very hard to remove (which is often as I re-do many of my figs). GW's skull white produced mostly just a powder substance (tried two cans to ensure the initial one wasn't bad). GW's black primer, as far as I'm concerned, is just black spray paint.
So, I recently read a post where Anders Heintz suggested Brite Touch auto primer and I decided to give that a try, which I purchase from O'Reilly's for $3 a can. I've used it it mostly on 90mm and busts, and just recently on a 54mm. I love it. It doesn't drown out detail and goes on like a second skin. Warm the can in warm water for a few minutes, and build up using very light coats (shake the can as often as you can during the process) I highly recommend this, or at least highly recommend that you give it a try. I haven't tried other brands of auto primer, but I'm sure they're all mostly the same.
Hope this helps some...
-Ronnie