We usually hit up Coral Pink a couple times in the summer. Coral is best for bikes and quads, but we've brought our rails there before and had an absolute blast. I love it there even for the cars. You just can't go on all the trails because some are pretty narrow and only for bikes, but only a few are like that.
I am hoping, depending on the cash flow this summer, to be able to make it up to St Anthony. I have heard nothing but fanatastic reviews of those dunes.
Even Sand Mountain (Little Sahara) is also a place I'd like to check out one of these days. I don't know how it compares to other places. I have heard the dunes aren't too great for actual duning, but I don't care. I like checking out new places no matter what.
I would never bring my rail to Pismo (Oceano). Even when you WD40 your frame you still get lots of rust and your car ends up looking 5 years older. Ask OGP about his Tatum.
Winchester or Coos bay has always been a cool place I'd LOVE to go to, but again I think they are better for quads. Besides, I couldn't imagine dragging my hauler for 17-19 hours and then having to do it all over again to come home. That drive alone turns me off completely.
Mark if our group does any trips this summer I'll let you know. You are welcome to hook up with us for a Coral trip or wherever we go.