For me, my only must own 22 is my Marlin Original Golden 39M 22 s,l,lr. This is kind of newer version of the Mountie made in the 70's and 80's but with a slimmer forearm and more tapered 20" barrel. So about 36" long and around 5 pounds, but solid as a rock.
The ultimate utlity rifle. And shoots everything well. Must be the micro-groove?

The other brands of lever actions are ok, and I'd take one if I had to. But none equal the Marlin to my thinking. Not the Winchester, Browning, or the Henry.
I've had a Remington 121, which was nice, but not the same durability or utility as the little straight stocked lever. Autos are fun, but none with the class or durability as the 39. I wouldn't mind a nice bolt action, but I wouldn't get rid of the 39M to get any of them.
Always had a hankerin for a 552BDL and a 597Magnum though.

Some kind of 22 pistol is good to have too. But I don't think any certain one is THE must have.
My Ruger Mark II Target Tapered is accurate, bulletproof, and a good choice. But I don't feel so strongly that another nice 22 pistol couldn't replace it. I could live with a Single Six, Single Nine, a K22 or any similar high quality double action revolver. A Colt Woodsman or various High Standards or Smith & Wesson autos. Browning Buckmark, something from Baretta, Sig, maybe Walther, or ever an SR22 could work for me. Or those mini-1911's are cool too.