Huh. I really love all of my knives and I don't regret any of them in the least. Most of these also have sentimental value, because most of them were for some sort of occasion (birthdays, Xmas, Father's Day, etc.) or a couple were random gifts from the wife. So I'll try, but this is hard.
Delica 4 HAP40: Surprise Xmas present from the wife; keyword is "surprise", which gives it a bit more sentimental value to me. Also, it's a Delica, in my favorite steel.
Dragonfly 2 HAP40: My first Spyderco was the ZDP DF2, and the design hooked me on Spyderco but good. But, I like HAP40 better. Either one could go here really.
Para Military 2 CPM CRU-Wear: It's a PM2, in what is rapidly becoming one of my favorite steels.
Techno: This was my first "nice" Spyderco. I fell in love with it the first time I held one, and lusted after it for a while before my wife bought it for me for Father's Day '15. It's still such a gem to carry, and now that I bought a KME I can really re-profile that edge down and have some fun with that.
Nirvana: This was a hard pick for #5. I managed to not pick any of the birthday knives (DF2 ZDP, Chap, Mantra, Sage 5) and I also could've picked the Sage 4 (which was a random surprise from the wife). But, it's the Nirvana, which makes it special on its own. I really enjoy carrying it, and it's just so nicely done. Also, this is the first one that wasn't
a special occasion knife. That's special to me because it was the first one (and what a first one) that I acquired just on its own, for no other reason, and I was able to afford to do so financially. The wife and I are only in the last year to 18 months really getting truly stable financially (we just bought a house a couple of months ago, for example), and it means something to me that I was able to buy a Nirvana of all things just because I wanted it and I could. I bought a CRK Inkosi at Blade this year (thanks again to the wife!) in a similar fashion. You guys probably know of those "you've been pre-approved!" credit card offer letters, and when your credit isn't great like ours wasn't a few years ago, they come with terrible interest rates and no real perks like cash back or whatever. But as your credit improves, those letters improve, and I for one sortof enjoy getting them now for the satisfaction of knowing how far we've come. So too do I get a satisfaction from carrying the Nirvana unrelated to most other reasons for enjoying it.
Sorry for the wall of text.