It really depends on your budget and planned uses for the rifle. For a general purpose/hunting rifle, I'd go with a 3-9x40 or 2-7x35 variable scope. The recommendations that follow are for a hunting/general purpose rifle.
Things to avoid:
Cheap scopes - Optics is one of those areas where you generally get what you pay for.
Doodads - You do not need target turrets, mil-dot reticles, 30mm scope tubes, illuminated reticles, adjustable objectives, or "tactical" anything, unless you absolutely know without a doubt that you want/need it for the type of shooting you are going to do and will use it. A lot of people pay a lot extra for these doodads and never get any real use out of them.
Over-scoping - Many people seem to think strapping an astronomical observatory to their rifle is the way to go, but a $300 plain-vanilla 3-9x40 scope will have better optical quality and better resolution than a $300 scope with higher magnification. It will be a LOT better than a $300 scope with high magnification and assorted doodads. Also, heat wave distortion can be an issue with higher magnifications. A clear, bright, sharp image at 9x beats a hazy image at 16x any day.
For a good scope on a budget, you can't beat the Burris Fullfield II. You can get one for $150 to $175 if you shop around. This is the scope I put on my daughter's deer rifle when I wanted a good scope that would enable her to enjoy hunting but wouldn't break the bank in case she decided hunting wasn't her thing.
If you want to step up a notch, the Nikon Monarch, Weaver Grand Slam, Leupold VX II or the Bushnell Elite 4200 scopes are all good choices and should be around $300. My personal choice in this range would be the Bushnell Elite 4200.
If you can afford a bit more ($400), the Zeiss Conquest is a superb scope with outstanding clarity and resolution. You would have to pay almost twice as much to find a scope with better optical quality. This is what I have on my deer rifle. It is the best scope I have ever owned, and unless I suddenly become rich enough to afford a Schmidt & Bender or Zeiss Diavari costing $1300+, I doubt I will ever feel the need to upgrade.