I have done lots of focus stacking. Free e-books on this at
http://macrostop.com/I use Nikons (D3x, D3s, D7000) bodies. I have four focus rails, and the best of the lot is the old Minolta that turns up on Ebay fairly often. However, I seldom use rails.
Instead find a macro lens with a long focus throw, 360-degrees or higher. Examples are the Voigtlander 125mm f.2.5 APO lens and the Leica 100mm Elmarit R lens (converted to Nikon mount). The new Nikon 105mm VR has a decent focus throw (270 degrees). Lens with a very short focus throw (90 to 240 degrees) are very hard to use because the tiniest movement of the barrel is often too much. A lens with a long focus throw allows you to turn just a hair and shoot, and then another, etc.
Here I am talking about nature macros that require anything form 12 to 50 layers, and not studio stacking where you may want 150 layers. For studio work, use a rail.
A longish focus throw is just as good as a rail IF you use a sturdy tripod and head. You can see scores of examples at the above mentioned link. The second book listed has specs on over 40 macro and close-up lenses, including their focus throws.