The Ultimak rail would be nice since it is the lowest one available, and is low enough to get a co-witness with a tube-body red dot in a low ring, and possibly with some of those mini red dots out there. If you want a cheaper one, CMP sells one that is made for them by Amega, and IIRC is only around a hundred bucks.
I would also consider a Scout scope, as has already been suggested. That is by far the cheapest, easiest, and most effective way to mount a scope on the M1. That's what they should have done in the first place with the M1 C/D!
Either way it will be a fast-sighting setup... it's just whether you want it more optimized for close range, or medium range. One thing I would watch out for on the red dots is dot size. The M1 is an 800 yard rifle, and even a 4 MOA red dot would completely obscure a man-sized target at 500 yards. Lots of them have an even bigger dot, like 7 or 8 MOA. THen it's as big as a dude at 250-300 yards or so. I guess if you were co-witnessed, you could just use your irons at longer range, but still, if I went with a red dot on mine, I would go with something with a 2 MOA dot, like some of the Aimpoints have. I am personally liking the Aimpoint PRO, and will probably be picking one up before too long.
And, I'd like to try an alternate sight system 
