depends
Depends alot on what type of rifle, and what you'll be shooting at. And....I'll say right up front, unless the rifle is a dedicated heavy varminter or match type (F-T/R) rifle, I'm not a fan of a lot of X power past 12 on hunting rifle. While I'm at it, it seems these days that the 4-12x, and the 3.5-10x, is quickly replacing the 3-9x as the GP hunting scope in my neck of the woods.
BAck to my objections to big glass. One reason is size and bulk. I've got a Burris XTR 6-24x on a match rifle, (.30) but the thing is akin to the Palomar telescope and would be all out of place on an AR carbine, or my tidy Mini-Mauser. And the more X you have cranked in, the more you need some type of support to shoot from or the perceived wobbles will drive you nuts. Thus either a bipod is attached (more weight and bulk) or one begins to get limited in how and where you shoot the rifle (or you dork with the scope).
I'll give a +1 to the low end variables on the AR carbnines. A 1-4x, or, a 1.5-5x, works really well for me and bamaboy on an AR carbine, both in 3 gun, and knocking around the back 40. I put a fixed 6x on the mini-mauser, but honestly, it is NOT enough X power for it and the intended smaller pest type targets out to the rifles max range, so the little bolt rifle is getting a 3x9x40 in the immediate near future. A compact model would be ideal, or even a model with 36mm bell, but the 40mm is on hand.
If I were setting up some type of urban counter sniper, or DMR rifle, say for an SO or PD, I might consider a fixed 6x with turrets, and a piggyback or offset dot for up close. Not really my baliwick though.
As a general rule, the bigger the rifle, the more willing I am to clamp on a bigger scope, but I do not want to spoil a tidy, portable rifle, with a huge telescope.