Underappreciated sets

[BobC]

I agree with all the sets mentioned so far; Delong, Oxford Confectionery, Diamond Stars, and the T205's (though the T205's aren't really unloved as much as they're more like the kid brother to the T206 set and just don't get as much attention as their big brother).

I'm going to nominate a few more sets as well, mostly for their uniqueness and in some cases first ever attempts at a somewhat new type of collectible.

1909-13 M101-2 Sporting News Supplements

1916 BF2 Ferguson Bakery Felt Pennants

1937 O-Pee-Chee Stand-Ups

1910-11 S74-1 White Silks

1911 S74-2 Colored Silks

And for a post-war nominee:

1948 Swell Sports Thrills

And postcards are great collectibles also, but aren't a set, they are more of an entire type or genre of a collectible, and the OP specifically asked about sets. They are getting more and more attention and love though as many card prices have risen so much over the past couple of years, lots of collectors may be turning to PCs as a more affordable alternative.