An Airlogic Genesis?,a winner if awards are given for weight.
Only handled one briefly but the Sheridan Blue Streak does have a certain charm.
Of the ones I’ve actually owned, and including single-strokes, in descending order:
Titan Mohawk
PH Dragon, when it worked (bloody unreliable thing, mine)
Sheridan C9
Crosman 120
Crosman 2200W
Tie: stocked Crosman 1377 and Crosman 2200
Crosman 760 (nearly forgot I’d had one!)
Daisy 840
What do you want to do with a pumper?
FT, HFT, 10M match none of the above. Proper pest control, the top two, though the C9 isn’t bad and nor is the 120. Outdoor plinking and occasional pest at moderate ranges, any of the C9, 120, and 2200s. Youth training, the 1377, 760, or 840. Indoor target practice, including bell target, any apart from the Titan and PH, with the multi-pumps on 1-3 pumps.
You are right that there are fewer pellet types available in .20”. But they are all good pellets, whereas 50-75% of available .177” and .22” pellets are mediocre to awful.