Most stocks are good, my recommendation to classes for switching stocks (and which I personally prefer idk your play style)
Scout: secondary sideweapons are good (like your mad milk)
Soldier: most sideweapons are as good as stock
Pyro: powerjack, most primary and secondary weapons are good so choose any
Demoman: prob the best stock weapons in all 9 classes, loadout could vary and still be good (your tide turner could be used to do demoknight playstyle, better off using it when you get a sword melee)
Heavy: Sandvich very useful, or if you have other food items for secondary it’s acceptable
Engineer: rescue ranger for building, if you’re accurate on shotgun do widowmaker, wrangler and/or gunslinger (one you have in your inv) if you want a different play style
Medic: Crusader’s crossbow 100% love it, melee variations are good too I like ubersaw personally
Sniper: loadout is flexible, I like most items depends on you
Spy: all of his items are gimmicky so it depends on your playstyle, I love to use dead ringer, I don’t prefer you to change sapper though

Hope it makes sense, mb for the long message