I'd have to say the Skyline is my favorite of the three. Here are some points to consider:
Scales: Winner...Skyline. I prefer the Skyline G10 scales over the Zancudo Zytel (I think they call it G10, but it feels more like Zytel to me...) and the metal handle of the Leek. The grippiness is just right.
Blade Shape: Winner...Skyline. I prefer the spear point blade of the Skyline clearly over the very fine point of the Leek...in fact my Random Leek is actually my favorite Leek just because it feels beefier. The Zancudo Drop Point blade shape is fine, it just feels a little stubby when compared to the Skyline of similar shape.
Deployment: Winner...Skyline. BUT, your mileage may vary. I don't care for assisted knives so the Skyline will always be ahead of the Leek for me. I tend to like flippers and once you get used to the Skyline flipper, it flies out. The Zancudo thumbstuds have always felt a little clumsy to me. It's one of those knives I have to focus on to deploy cleanly.
Lock up: Winner...Zancudo. I guess I'm one of those idiots that blindly accepts that a frame lock is stronger than a liner lock with no real reason to support it. In fact, the frame lock on my Avispa (Zancudo's big brother) has shown some lock creep with VERY light use...so really I don't know. In general, when given the choice between Frame and Liner, I'll choose Frame.
Clip and Carry: Winner...NONE. Honestly, all three knives, for my tastes, have horrible clips and horrible carry. Now, in their defense, they all do offer tip up/tip down as an option, but none are friendly for the lefty. All three clips sucks, in fact I think Zancudo and Leek use the same darn clip. Replace the clip as soon as possible with a nice, deep carry option...unless you like a goofy, pointed bump sticking out of your pocket, in which case ignore this comment and enjoy your goofy, pointed bump.
Blade Steel: No comment. I'm remarkably unskilled at sharpening so I'm not even going to try to discern between 14C28N and AUS8. Let's just say that they're both adequate for EDC tasks from my perspective.
Value: Winner...Skyline. You'd have to call the price of the Skyline and Zancudo a wash, they can usually be found for between $30 to $40. Given the similar pricepoint, I think the Skyline with somewhat superior materials edges the Zancudo. The quality of the Leek and Skyline are very similar, but a Leek will commonly run $30-$40 MORE than a Skyline.
Care and Maintenance: Winner...Skyline. My Skylines have always disassembled and re-assembled without a hitch. Sometime my Zancudo has seemed to not go back together quite as easily. Although the Leek is well made, assembling a knife with a torsion bar can be tricky if you've never done it before.
Origin: Winner...Three Way Tie! All originate from the planet Earth. Yay Terrans!
Variety: Winner...Leek. Seriously, there are so many flavors and varieties of Leek, some days I wake up thanking the Powers That Be that I never decided to collect them. If you want a pink handled Leek with a rhinestone unicorn sporting an acid washed damascus blade etched with a glowing dragon eye...they probably made it and it can be found on the Evil Auction Site. It's the Kershaw 1660DAMPKUNIEYE.
Anyway, that's my two cents from someone that owns and has carried all three. Honestly, they're all great knives and whichever you choose you can't go wrong. Besides, you know you'll end up getting all three...that's how it is with knife-love.
