The wrought ones mentioned by Russ are really nice... sometimes you'll see one that's been beat so hard for so long that the face has been sagged in the middle due to the soft wrought underneath having been upset a little. The other thing to watch for in those is with some that were built up by forge welding big chunks, the heel or horn may have a weaker weld and should not be hit with maximum force, particularly when cold. They can break off... not hard to avoid doing. Some will have super hard faces, which is good except for in cases such as some Kohlswas that have chippy edges.
Speaking of which, there are cast iron anvils with tool steel tops- the Fishers are excellent anvils with a lot of rebound, but chippy edges sometimes as the top can be quite hard. The Vulcans work too but it's common to see them with chipped or broken off faces as their process and quality was inferior to Fisher.
Cast steel anvils with hardened tops can be great, but the tops will sometimes be a bit soft- an examination of listed Rockwell hardnesses for available new/modern anvils shows a range of high 40's to mid/high 50's Rockwell. I'd try to find something with at least mid 50's for a top, if shopping for new. I don't like a top that will dent significantly from a light hammer strike - for one, the rebound is not great then, and for two if you ever have students over they will ding your faceplate up.
My favorites are old wrought based ones, Trenton, Arm and Hammer, Hay Budden, Peter Wright, Kohlswa and Paragon. My almost top favorite though is Fisher, the cast iron/hard top - great rebound and no ring.
