There is no simple answer as fundamentally the final choice depends on your eyes (and wallet).
The scope will need to have a good depth of field so avoid objective lenses of over 44mm and magnification of more than 10x
The reticule will need to be easy to read - the commonest and most popular is a mildot or better still half mildot which will allow you to hold over or under more accurately / easily.
Scope height is again personal taste - low to medium mounts are usually the best way to start then experiment as you get more practised.
Get a scope that is parallaxed at airgun ranges ~30yds or better still has adjustable parallax
I am currently using the Viper Pro 10x44 which works for me, normally we would advise trying out a number of scopes before buying but that is not so easy at the moment so start with a mid range scope - very cheap optics struggle in very bright / high contrast conditions and low light conditions but it may be worth looking at SH kit on this site or have a look at your local gun shop or somewhere like Optics Warehouse. Many of the Hawke scopes are a good start point.....
OAKS - SIHFT Winners 2011, [SIZE=4][COLOR="#FF0000"] 2012, 2013, 2014, 2015, 2015, 2023