Evenflo Maestro would be the cheapest solution - at $80. Good harnessed seat, but once she outgrows the harness, it won't last as a booster for very long. So you'd be buying another booster at some point.
Graco Nautilus would be perfect - $150 or less. GREAT harnessed seat, easy to install and use. Then becomes a terrific booster that should last a long time. And becomes a backless booster as well.
Britax Frontier is on the pricier side, and they just updated it to be the new Frontier85. Will harness longer than any other seat, and then is the tallest booster currently available. Big heavy bulky seat, but has some great features, like the no rethread harness. The original Frontier can be a pain to install, but that has supposedly been made better with the Frontier85.
Since car seats don't actually receive ratings - just pass/fail, any of those would be equally safe as long as they fit your child, your car, and will be used correctly 100% of the time. If its hard for you to use, you won't use it right and then it can't do its job.