I know Colin's very fond of the ASG, I'm also very fond of the VFC Glocks, and just bought CO2 mags to go in them. Upgrades are plentiful, and just a barrel + bucking will give you a good improvement for very little money. Running an 18c full-auto on CO2 is great fun!
Other than that i have a CO2 KWC/Umarex USP which i really like but there are no off the shelf upgrades and the trigger bar is a well-known weak point. Shoots really well and a strong sharp kick.