The power hammer you should buy depends a lot on your budget, experience level, and what you want to do.
Mechanical power hammers
a little giant or treadle hammer is better than your right arm or left arm. one tire hammer also will be a not good choice.
Self-contained power hammer
That being said, I would only have a self contained air hammer, no external compressor hammers, no mechanical hammers. this type hammers will save much space and with good controllable. anyang ST power hammers always is welcomed.
In the self contained market, my first choice would be an new hammer with good performance and warranty. the cast frame will better than the fabricated designs, the cast frame air channel is cast in the metal, it’s volume will keep same when it works. But the fabricated design air channels are made from rubbers, i will not be shrink or dilate when it works, it will influence the air volume then not good for controlling performance.
about the second hammers, old Nazel will be the first choice. Second, Massey clearspace. Third any of the following in no particular order; choice the old hammer will have to consider the repair or maintenance service.