I'm not a collector. For me,knives are tools, so I don't consider any one particular brand or model a 'must have'. I wouldn't tell anyone Snap-on was a must have, but I would consider a decent 1/2" open/box end wrench a must have in a tool box. So my must have list is based on style of knife.
My list would be:
A good 2.5" - 3.5" modern folder - this might be the most used knife in anyone's "tool box" of knives. It's always with you in your pocket, gets pulled out for duty on an almost daily basis, whether slicing an apple, cutting rope, breaking down boxes, or just opening a letter. Everyone can find a use for one, no matter what your profession or daily needs are. Mechanics, plumbers, hunters, hikers, stock boys, desk jockeys, and everyone in between - they can all benefit from a good, modern folder in their pocket. Doesn't matter if it's a $600 Sebanza or a $70 Leek, it'll get used fairly regularly. Traditionals will work too, but a modern, one handed folder will serve a wider range, since often, your other hand is busy holding something.
3"-4" drop point fixed blade. Again, it's a tool that has proved its usefulness time and time again. Small enough to not be overkill for most tasks, big enough to actually do most tasks. Hiking, camping, hunting, farming, or just working in the yard, a fixed blade in this size will do just about anything that a 5" or larger will do. If you need to chop, get an axe. If you need to clear brush, get a machete. If you need to cut, slice, thrust, etc, a 3.5" blade will do 95% of the knife tasks you will ever need.
A good multi-tool. When your toolbox isn't within reach, you can improvise a lot with a multi. You might not be able to rebuild an engine, or raise a barn with it, but when you're out in the field, and need repair in a pinch, a Leatherman Wave will go a long way.
Finally, a good hand axe. If you gotta chop, nothing beats it, and you can, arguably, even hammer or baton with it. Nothing breaks down large material like an axe.
Everything else is all specialty tools. Not must haves, unless you do those types of jobs regularly.
- A large fixed blade is nice, but there's not really anything you can do with it that can't be done with either a smaller fixed blade or an axe.
- A mechete or Khukri is very useful if you're in the bush a lot, or need to clear land by hand, but for urban dwellers and suburbanites, even those of us who are outdoors a lot, hunting, fishing, or camping, it's a very specialized tool that won't get used much.
- Lots of people would list an SAK as must have, but for me, a multi, even a small one like a LM Squirt or Juice, surpasses it in functionality. Think of it like the difference between a traditional vs. modern folder. Same purpose, but with a more updated tool-set and functionality of design.
Like I said, I see knives as tools, first and foremost. What I consider must-haves, are based on what I find most useful on a regular basis. Though they're in my toolbox, I can live without an impact gun, torque wrench, etc. - but a 1/4" ratchet, some basic combination wrenches, and a philips and straight screwdriver take care of 75% of the things I need to do.