Tastes are subjective, and what may appeal to some won't appeal to others. For example, I hate spay blades. I like the stockman pattern, but if it has a pen blade rather than a spay blade, I'll like it 100 times more. I also don't really care for locking traditionals, or for that matter any single blade traditional. If I carry a locking blade, it's going to be a modern folder, with 1 hand opening and a pocket clip. And if I carry a traditional knife, it's going to have more than 1 blade, and at least one will be curved, and one will be straight. To me, the advantage of carrying a traditional pocket knife is having multiple blade types in a small package. Something like a Buck 110 or slimline trapper, although I can appreciate them aesthetically, will never find their way into my pocket.
That said, when it comes to "must have", only 2 specific knives come to mind. A Victorinox Soldier/Pioneer, because it's almost infinitely useful. Until you've carried a knife with a quality reamer you'll never realize just how handy it can be for handyman, around-the-house/shop tasks. And it'll open your beers when the day is done. The other knife is the Case Sway Back Jack. This is based more on my opinion than empirical usefulness, but it's just so damn nice a knife that I believe everybody who likes pocket knives should at least handle one, if not own one.
It's also my opinion that everybody should have a stockman, whittler, or congress with at least 3 different blades - whatever their preference. So many people, even knife enthusiasts, don't know how useful a 1" long sheepsfoot blade can be. If attacked by a rampaging gang of clamshell packaging, I'd rather have the sheepsfoot blade of a humble stockman than a Sebenza, Strider SnG, or Lionsteel SR-1.