I am not a trader/seller and have way too many for varied occasions to pick just one, but for pure hunting memories and more game down than I can ever recall, I still own a 1975 purchased M700 in 30-06 with a close second 1960's model Belgium FN in .270Win. Both of those have been used to kill everything from crows, coyotes, bobcats, deer, elk, etc, etc. Before I ever considered owning a magnum rifle, I could shoot these out to 1/2 mile and occasionally beyond. Those rifles became an extension of my arm/hands and have snap shot many fleeing animals. Now for LR/ELR large game, I use a couple of 300RUMs.
In the varmint or so class, I would say my old 1979 M700 in 222Rem has taken more varmints and up to deer sized game than any other varmint rifle I own. Several barrels have lost their throats to that old action, and countless thousands of varmints and game. It saw many 3-500rd days on the dog towns, but now with age and far less shooting done, I like something with a little more reach and less drift. So, I take out a 22/250AI in 8" twist whenever I can.
It is interesting to think back on this question and realize that all the newer, fancier, faster and most costly customs and some production rifles do not jump into my memories as a "favorite" like those old well-worn trusted friends do.