Photoshop 5 - Helicon focus - CombineZP - Zerene Stacker
Those are the options on the market that I am aware of at the moment and in general there isn't a market leader. So very much like noise reduction - sharpening and many other editing processes no single program beats all the others; whilst each one will have slightly different controls and effects.
However with focus stacking software the slight differences can be the difference between a stack that works and one that totally fails. Based on the larger investment in time and effort in getting a series of shots for stacking even before getting to editing I, personally, think that its worth (over time) building up and learning to use the whole series of options. Chances are one will become your main use, but for specific situations you'll have the others to draw upon. Everyone can get at least one if not two (CombineZP is legally freeware so no costs attached) and if you are working for home use only the fees on the others (barring PS5) are quite reasonable.