Like was stated, it's difficult to quantify what makes them worthy of "top 5". I think it's a balance of quality, price, and popularity.
1. Buck 110 - Without the influence of the Buck 110 the knife world as we know it would be massively different. It's a mainstay today as much as always, although maybe not among the more discerning crowd.
2. Spyderco Delica - Spyderco pioneered the pocket clip, and without it the knife world would be very different. The Delica is regarded among many as being the perfect EDC and is probably more prolific than the PM2 due to economics.
3. Benchmade Griptilian - This knife is the every man's Benchmade/Axis lock entry point. Not the most refined or best knife Benchmade makes but it's affordable, good quality, and well known.
4. Kershaw Leek - Regarded as one of the best EDC knives out there. Also, it has the iconic speedsafe technology, if you're into that kind of thing.
5. Kabar - mention a combat knife and what image comes to mind? The Kabar. Not the only maker of the knife but one of the finest few remaining. Ontario makes good users and may arguably be better.
There are so many it's hard to stop at 5.