For me, it's mid '80's GMC diesel conversion vans... they are cheap... cheap to buy, great mileage for what they are, good reliability and awesome comfort... they've got captain's chairs and a queen sized bed in the back!
I've got two of them, both have the non-turbo 6.2L Detroit diesel, STAY AWAY from the earlier 5.7L GMC diesel. If you need more power than the 6.2 produces, go for the later 6.5L NA or the 6.5TD motor, it's the same block as the 6.2 but different heads (and it's the same motor as the H1 Hummers) but it's rare to find a turbo diesel in a van, they don't fit in there all that well. There are a few different types of turbos for the GMC's, the center-mount turbo motors that H1 Hummers have DO fit.
I keep track of my mileage religiously, (like EVERY tankful) and I get average 20.5mpg day in and day out, whether I'm going to Seattle or just back and forth to work in Sequim... they are my daily drivers. Towing my 19' Custom Weld, I get 15mpg. I've even tried biodiesel, and in my rigs, I get about 1.5mpg less with b100.
My first GMC van I bought with 84,000 miles on it, it's now got 320,000 miles on it and needs the heads resleeved. My second van I bought with 144,000 miles on it (just broken in) it now has 159,000 miles on it and is going strong... the 6.2's and earlier 6.5's use a inexpensive and reliable mechanical injection pump, you need to put an injection pump in them every 150,000 miles or so ($325), and injectors every 80,000 miles or so ($25 each)... compare those prices to what you'd pay for the same parts on a PowerStroke or Cummins... granted, those are different motors with lots more power and worse milage... but if you don't need it, why pay for it?
My dream? To convert my original GMC van over to 4WD... some day...
-N.