I've owned most of 'em...
... and, FWIW, here's my ranking:
1. Benchmade Mini-Griptilian: $60
A $100-knife-for-60-bucks-bargain. Slender but rugged. Disappears in your pocket, too. Axis lock is fast, proven, and super-reliable. A big seller. Pick one up and you'll understand. 440C (not 440A) steel is very good, sharpens fast and holds an edge well. Great Benchmade company "genes" and customer service. Of its different versions, the plain-edge/thumb-stud opener (model 556) is my favorite, and often "out of stock." It's THE ONE I'd go to war with in this price range.
2. Spyderco Delica: $50 +/-
All-time best seller. Good value. Great VG-10 blade steel. Thumb-hole opener doesn't work for everybody. Lightest weight (1.9 oz. w/the plastic version; the stainless version is heavier, costs more, and has slightly inferior AUS6 steel).
3. Kershaw Leek: $50
Nice sleek design. Average 440A steel is OK for moderate tasks. The most "sex appeal." Some think the assisted-opener is a less-than-durable boy-toy gimmick, and it does break. Excellent Kershaw customer service, however.
The tiny Kershaw Chive is "cute" but has so-so 420HC steel and is hard to open under stress when your fingers turn to clumsy thumbs. If you're really hot for the Kershaw assisted-opener deal, consider their new Mini Mojo ($65) with its 440A steel and improved speed-safe device. It's kinda like a Chive on steroids.
My advice is to go to a gun show, play with 'em, then go with your gut. FWIW, my favorite on-line sellers are New Graham Knives and Cutlery Shoppe.