Boardwalk and Saratoga, both easily above all other choices for me. I've stayed solo at other resorts too - I mean, I'm at Disney, so it's never a bad time. But Boardwalk's access to 9 other resorts via walking, boat, or Skyliner, plus walking access to Epcot and DHS, make it a definite favorite - plenty of restaurants, bars, and easy park access. Saratoga is a close second - specifically if I stay in Congress Park. Walking access to Disney Springs provides dozens of restaurants and bars, and walking to the Disney Springs bus stop provides easy one-stop access to EVERY other resort in Disney World. Plus the lovely boat rides to OKW or Port Orleans.
My next solo trip is the first week of February at the Fort Wilderness Cabins - I know access will not be as convenient to parks and other resorts, but I will have a golf cart making it easier to get around, and down to the boat launch, where I can get to Wilderness Lodge, Contemporary, MK, and the MK resort area easily. I have soloed at OKW, Boulder Ridge, AK Jambo, BLT, and Beach - all were fine stays and some like Beach share the same advantages of Boardwalk...OKW has boat acess to Disney Springs which isn't quite as convenient as Saratoga, but works in a pinch. Probably my least favorite for solo was AKL, just because there's no walking access to any park or other resort, and all transportation is bus-only - my least favorite method of transport.
I may be an exception because things like pools or spas don't matter to me - I never use resort pools (living in S. Florida, in a beach town, with a big pool in my backyard, means I just don't have any draw to pools when on vacation). So for me, it's all about access to food and drink, entertainment, parks, and good walking access.