The Leupold 6.5-20X has all the bells and whistles: sunshade, turret-mounted parallax adjustment and target knobs.
You can build or buy the perfect varmint rifle, but you're wasting your money if you don't scope it intelligently.
Variable scopes excel for varmints. Shooting to the far edge of a prairie dog town, you can use 20X to advantage. But when dogs pop up closer or mirage runs heavy, or when you're hunched over a coyote call, 10X might work best.
High power shrinks field and exit pupil, impairing your ability to find targets quickly and in poor light. If you shoot from field positions with your varminter, you'll want no more than 16X. Ditto if the wind is bending the barbed wire.
On days when I pretend yon 'chuck is a bighorn ram, I dial down to 6X. In sum, you'll enjoy a 6-20X scope. No need for more power, and you won't want much less.
Big objective glass has little to recommend it. I prefer 40mm because it accommodates low rings and keeps scope weight reasonable. Yes, most varmint rifle stocks permit comfortable aim through medium or even high rings, which you'll need for 50mm objectives. Whatever the size, get a sunshade to assist vision when glancing sun tries to veil your view.
As for tube diameter, I prefer an inch. Until you shoot beyond 600 yards, you won't need a greater range of adjustment than is available in one-inch pipe. On the other hand, a 30mm tube can complement the balance and aesthetics of a varmint rifle.
You'll appreciate target knobs with positive clicks and resettable zeros. You'll adjust a varmint scope frequently. And because you won't carry it through thickets, tall knobs are no liability.
An adjustable objective eliminates parallax at selected ranges; it refines focus too. Choose a turret-mounted dial rather than a front sleeve that forces you out of shooting position when you adjust it.
Some varmint reticles are so fine they're hard to see, especially against variegated backgrounds. I prefer a standard crosswire or a crosswire and dot. If you'll be hunting coyotes in the sage, pick a heavier wire than you think you need.
For the most part, rangefinding reticles leave me cold. A scope's field should be open. Darrell Holland's ART (Advanced Reticle Technology) crosswire, with both m.oa. and mil-dot tics, has minimal clutter. The laser rangefinding scope by Zeiss is an excellent alternative.
Cinch your mount screws firmly; tighten ring screws evenly and with less enthusiasm. Make sure that the reticle is oriented properly and that there's enough eye relief for shooting prone. Even a .22 can bloody your brow.