I'll start it off. Maybe these lists will give newcomers some idea of what guns are worth looking at regardless of age, new or used.
.177 - Daystate Red Wolf Standard
.20 - HW77k
.22 - KalibrGun Cricket Mini Carbine
.25 - RTI Mora / Huben GK1
.30 - Daystate Red Wolf HP / BRK Sonoran
.35 - AAA Slayer
.50 - Umarex Hammer Carbine
Of course there are others in some calibers which rate very highly, but this is the current preferred rotation.
.177 - Daystate Red Wolf Standard
.20 - HW77k
.22 - KalibrGun Cricket Mini Carbine
.25 - RTI Mora / Huben GK1
.30 - Daystate Red Wolf HP / BRK Sonoran
.35 - AAA Slayer
.50 - Umarex Hammer Carbine
Of course there are others in some calibers which rate very highly, but this is the current preferred rotation.