I use a 4x40 Tasco and I'm not planning to change it. It is really good, the only thing I would change are the turrets, not these ones where I have to unscrew the caps and then use screwdriver to adjust them. If you don't plan to spend around let's say £80+ then go for a fixed zoom scope. It is much brighter and clearer than the cheapish variable zooming ones. Anything between 4-6 magnification is optimal for airsoft. If you want it for a bolt action sniper rifle then 40mm is good, 50mm is too big, 32mm- might be OK for DMR.
If you want to spend more then a Leupold M3 replica is very good I heard.