Your eyeball and the little metal thingy's on the barrel.
There’s always an Arquillian Battle Cruiser, or a Corillian Death Ray, or an intergalactic plague that is about to wipe out all life on this miserable little planet, and the only way these people can get on with their happy lives is that they DO NOT KNOW ABOUT IT!
Under $100 I'd go with a Simmons. Several different models to choose from, with the 22mag being very popular. I paid $120 for the scope itself, a Weaver RV7 2.5-7x28. It's a quality scope running with scopes that cost way more, and IMO 2nd to none for the $'s!
It may stretch your budget a tiny bit, but if you justified a CZ then...
I just put a Bushnell Banner 6-18x50AO on my Savage, with B-Square Sport Utility Rings. Seems to work quite well. It is made in Korea compared to many at that price level which now come from China.
What are your guys' opinions on Leapers brand scopes? My friend had one that he had on his air rifle and I was thinking of buying it off of him and putting it on my future .22LR. Its a red illuminated reticle 3-9x40 I believe.
Best all-around scope I have for rimfires is the 4-12X40AO Bushnell, about $70 at Wally's. Anything bigger is a waste unless you are into serious competition shooting.
The only downside of the Bushnell is the rather large eyepiece, that requires tall rings on some bolt-actions. The Adustable Objective gives a much closer 'minimum range' than fixed-objective scopes. Perfect for rimfires, where you may need to 'get up close and personal'.
Nikon Prostaff.....E-Bay.....Some really good deals on brand new, in the box. Just saw one go for 80 some odd bucks. I bought one from Sportsman's Warehouse for 150 bucks, or there abouts, and got one on E-Bay for $102 including shipping...new in the box. YMMV