I like the little SMB's for marking my position during routine deco - I use a Halcyon small marker most of the time. It's adequate for establishing a reference line, inflates orally or through an lp hose, is small enough that I don't have to worry about it wagging the dog, won't spill and is big enough to be seen in calm conditions.
I have a couple of back-ups that I carry (not usually at the same time): a 50# OMS SMB with a duck valve and an OPV in a pouch on the butt of my backplate, and a 50# Carter lift bag with an open bottom that rides under my backplate. The SMB is better for marking my position only if I'm on the surface and can apply sufficient downward pressure to lift it out of the water, the lift bag has an irritating tendency to dump air if I don't pay attention to it all the way up.