1. 1947-1955 GM Advance Design pickups
These trucks are fairly easy to find, so you can afford to be picky here. And you should be. Look for low-mileage short-bed half-tons without a lot of aftermarket chrome add-ons or modern bits. What you want here is originality over restoration, unless the restoration is completely stock and only includes what the truck would have had from new. Remember: most of these rigs didn't include a lot of convienience features.
What you should pay: Up to $35k, depending on originality and condition.
2. 1971-1972 Chevrolet C10/C20 Cheyenne Super
If you want an everyday usable truck for today's roads and still want that vintage look, it doesn't get much better than these highly-optioned examples. The Super featured things like special interiors with headliners, special paint and trim, and usually included options such as tilt columns and a/c. And starting in 1971, front disc brakes became standard on the 10s and 20s, so they offer decent stopping power. Find one with as many documented options as you can - look for a ’72 Cheyenne Super 4x4 short bed with a/c, power steering, power brakes, and a tach dash.
What you should pay: $15k—$30k, depending on options, originality and condition.
3. 1948 Ford F-1
This was the first Ford F-series pickup, and considering it's been America's best selling truck for years, that's nothing to take lightly. Again, like with the GM Advance Design trucks, I'd look for the best original example you can find, and leave the chrome add-ons to the custom crowd.
What you should pay: Up to $30k, depending on originality and condition.
4. 1946-1968 Dodge Power Wagon
The Power Wagon has an all-business go-anywhere 4x4 look about it, and they've seen quite a bit of upside in the market over the past few years. Designed in the early 1940s with the military in mind, these trucks didn't change much until they were discontinued in 1968.
What you should pay: Up to $45k, depending on originality and condition.
5. 1966-1977 Ford Bronco
These square Broncos have a huge following, and really good early examples without modifications are now tough to find, especially with low miles and no rust. Roadsters and half-cabs are rare.
What you should pay: Up to $25k, depending on originality, options, and condition.
What else should be on this list? Post it in the comments below.