There's some really well made post WW2 firearms in both European and American makers. A lot of this depends on your budget, of course, but the boutique makers in the US really took off post WW2. It also tends to coincide with the loss of handmade works out of the factories as production costs were being cut through the automation of manufacturing, rather than relying on skilled tradesmen & women. The Winchester model 70, and most of the Winchester line, pre 1964 are prime collectables that are often times affordable. I'd keep my eye out for custom rifles though. I picked up a custom .338 Winchester magnum on a P14 Winchester made action that shoots well under an inch at 100 yards, and has taken game cleanly out to 375 yards. I'm currently fitting that 1970's custom to a 1930's stock from an old Remington Model 30 Express that I picked up from
@Durango Mike. That rifle has the best of both worlds now, IMO. Classic straight grained American stock with a well built custom rifle. My Model 70 featherwight is no collector since it's been refinished & I added a red pad to it, but it's a fantastic rifle & I've taken the most game with it, out of all in my safe. My Tang Safety Ruger M77 was my first grown up rifle. It was in 7mm Mauser, but sprayed bullets, so now it's got a new tube in 7mm AI and it's a tack driver with family history.
Pre-64 levers from Winchester, JM stamped Marlins, etc all have collectability and they are excellent hunting weapons. You've also got some quality Japanese guns in this mix, like the Browning BSS, Winchester 101, etc. Lots of companies went overseas post WW2, especially to Japan, when they found out the quality of the build was better and costs lower than hiring, training & retaining skilled craftsmen. Then you have the Weatherby SKB shotguns that were imported from Japan as well - super quality guns and often times way undervalued for what they are in terms of build quality.
Post war European rifles & shotguns are well made firearms with a lot of collectability. I am always looking out for the right 1940's - 1960's vintage Browning Superposed. Steyr, Sako's, Mauser , Merkels, Simsons and Krieghoffs all hold their value extremely well and you can usually find some solid custom rifles that are not marked but were built for the trade, such as the FN Mausers used by JC Higgins - combining American ingenuity with old world craftmanship. Spanish shotguns are highly collectable if the right maker and model is involved, but you can still pick up an AyA Matador for under $1000 in fantastic condition, and the Italian guns from this period are solid collectors as well and can usually be found for a decent price. Husky's are usually undervalued, IMO. Stepping into British firearms generally means a significant investment in terms of cash, but most of those guns will hold or increase in value if properly maintained.
WW2 bringbacks can still be found and can provide a chance to get into a pre-war sporting rifle at post war pricing. I've got a Haenel made carbine in 8x57J that was made for Mouflon and Chamois hunting that had a note from the son of the GI who brought it back in '46 and a 98 Mauser that was somebody's cigarette gun during the occupation (when German gunsmiths were forbidden to make money on firearms, so traded cigarettes to US Servicemen looking to send home a trophy or two).
If your heart is set on that period, do a lot of research in terms of collectability value and possible investment value. Knowing your target piece is critical both in terms of assessing value and in assessing it's condition. Foreign guns might have bore issues from Berdan primers using corrosive powders, and American guns might just be beaten to hell and back.
Fit & Finish: Look for hand checkering, not pressed or lazer cut. Hand checkering has a better feel to it and it both precise and imperfect. Wood to metal fit is important as well. If a stock is too proud of the metal, then it's going to feel clunky in the hand. If the metal is proud of the wood, then you likely have a firearm that's been refinished. Check the quality of the finish, peeling of stocks, crazing of the finish, etc all reduce the value of the firearm. Pads should be from the era and still subtle and soft. If it's brittle then the pad will need to be replaced, which could affect the value of the gun. Metal finish too - be on the look out for patches of cold blue, rust, pitting and really scope out the bore. My Mauser had throat erosion to the point where I had 2" of freebore and the bbl was toast.
Refinished firearms are a lower collectable than factory original. Additions like corn-cob adjustable chokes lower value a lot, and recoil pads do as well. For factory firearms - original is the key. Even if it's beat to hell. Rough stocks with original finish> refinished. Patinaed metal>reblued metal, etc.
Customs can generally be had for less than factory originals, but it's not really about the monetary value they hold as much as respecting the craft of the maker and enjoying a finely tuned firearm that was built by hand.
I personally prefer sporting weapons between the late 1890's & 1950 from both Europe and North America. At the end of the day, this is like collecting art: You can do it for a variety of reasons, but the most important is your own personal aesthetic. This is a significant disease, though. So your best bet is to really dig into the history of the period you want to collect from and dial in what you want, become as expert on them as you can, and be selective in what you buy. Regretting the purchase of $1500 - $5000 item isn't fun.