Pathfinder is probably the most powerful Finder alternative out there, although it's a tad clunky in places. I find it useful enough to keep it on my system, though.
Forklift is primarily an FTP client, so not directly comparable to Pathfinder (which doesn't do FTP beyond the level of functionality already built into the Finder ). It can also be used as a Finder alternative, but unless you're particularly fond of the dual-pane view, that's not really its strength.
Alternatively take a look at Interarchy and Transmit, which are probably the best of the 'power user' FTP clients out there - if your needs are fairly basic, however, I'd stick with something like Cyberduck.
Leap: This is more of a search tool than a Finder replacement - it's essentially a Spotlight enhancer. It's very well implemented, but personally I find it rather a 'solution in search of a problem'. Feel free to disagree, however =]
Essentially, the applications you're talking about all work very differently from each other and don't really overlap that much - we're not comparing like with like, here. If you want a more powerful alternative to the Finder, then PathFinder is probably the way to go. If you're looking for other functionality, then that's a different matter.
Personally, I use Launchbar along with Pathfinder as my primary Finder alternatives/enhancements plus Interarchy for FTP.
