I've had a few really surprise me over the years, but most recently was my old 1993 Ruger M77 MKII 24" 7mm RemMag. Bought used in 2002 at the gun store I worked at. It sat on the shelf collecting dust for over a year because it kicked like a mule, and nobody wanted it, so I got it dirt cheap.
Back in May of this year I had my smith do some updates to it, like drilling and tapping the receiver for 8-40 screws and mounting an EGW 20MOA picatinny rail on it. He also did a full stone & polish trigger job, and lightened the pull to a nice clean & crisp 2 lbs. Zero creep or over travel. Also, had him thread the thin sporter barrel and permanently install a 5/8-24 shouldered adapter made from an old piece of 416r barrel he had laying around, so I can run my suppressors on it.
Now that I had a decent gun, and a good scope on it, I decided to work up a load for it with some old-stock Nosler 160 AccuBonds and some new old (still in the original bag) Winchester 7mmRM brass that I forgot I had sitting sealed up in an old .50 cal ammo can for the last 10+ years. First round of load workup, I found a nice node, and this was the best groups in that velocity node. It's not the fastest load at around 2,933 FPS, but it's a solid performer.
These were shot back-to-back (letting it cool, of course) on the same day, from the same set of load development. I'd say I found the sweet-spot.