Here's my hot picks:
--Swiss straight pulls. K-31's, 1911's, 96/11's and all variants. These were never made in large numbers and are all excellent shooters. The prices are already heading north fast. A few years ago it was very hard to find reloadable ammo, but that situation is improving very quickly. The 7.5 Swiss is basically a .308 Ackley Improved, though it was designed half a century before the .308. The Swiss were generations ahead of us. The round is astonishingly accurate. Loaded properly it can easily give sub-MOA groups. Don't buy into the rumors about weak actions or vulnerability to field conditions. The K-31 in particular is as strong as an M-98, and the 1911 is strong enough for GP-11.
--Finnish Mosin-Nagants. The pre-war variations are already getting spendy, but can be found if you look sharp on the secondary market. The importer market has some wartime and post war M-39's, M-91's and M-30's, but supplies are limited. Now is the time to buy these. Some of the subtypes are incredibly rare and all have a good rep as shooters. They're very different from Soviet Mosins. The 54R is an ancient warhorse of a cartridge and there's plenty of it to be found.
--South American Mausers. These are mostly limited to the secondary market, but you can still find some for a reasonable price. There's a lot of growing collector's interest in these and they include some fine rifles.
--Swedish Mausers. These are getting really hard to find. They're fast in the hand and hit a lot harder than the paper ballistics would suggest. They're very accurate, of course. The 6.5 Swede is probably the best deer cartridge ever made.
--Great War rifles. The WWII rifles get 90% of the attention, but IMHO the better choice for a collector is often the WWI rifles. These are mostly on the secodary market now and a lot of them have been hacked up by bubba or importers. But if you can find one intact for a good price, grab it up. These include the 98 Mauser G's, the Swiss 1911's, the Imperial Russian M-91's, Eddystones, the old SMLE's, and a lot of other long, long LONG rifles. If they're not hacked, grab 'em. You may even find an old American Enfield with an "EK" stamped on the side of the stock! I found one once but the owner was saavy to what that stood for and wouldn't sell.
--Weird old stuff. Keep an eye out for weird old rifles. Mannlicher-Schoenauers, Siamese Mausers, Austro-Hungarian long rifles, etc. My general rule is that if it's fully intact and cheap, it's a buy. I don't keep a C&R license because if I did I'd be awash in these things.