Quality glass, that is your main need like others have said.
As for magnification, as much as you can handle.A high power variable has lower magnification than the dial all the way to the max, lots of people forget that.Why handicap yourself when you might need more.6-24 , 8-32, 10-50 , not very often you are going to be using the minimums.I find myself between 20-40 with a 10-50 depending on conditions.
Make sure the scope has a good focus adjustment for your desired range.
A good focus and good glass is your keys to clarity.Objective size is relative to light gathering and does have some bearing in regards to exit pupil and magnification BUT , you are almost never shooting targets at dawn or dusk and this is where the big objective can be a benefit(again, glass quality being most important).
Also, you want to look at reticles, one that gives you enough marks for measuring and references but not thick and heavy to obscure your target.
In conclusion, I would get 8-32 or 10-50 , objective doesn't matter, 30mm tube for maximum adjustment and a scope that has quality focus adjustment.
You WILL get more fussy as you use it.