Best is in the eye of the beerholder. But I think works great Heather's will completely disagree.
Where I shoot I can only get out to 600 yards. Around 7 maybe 9 power depending on the day, sometimes 5. This is for typical range shooting, banging Steel having a good time Etc. If I'm doing load development then maybe I'm in the 12 to 15 power range
Now, if we're talkin about competitive benchrest shooting out to 600 yards then I'm looking at it somewhere around 40 power.
But what I really prefer is no power, iron sights. That's what starts separating those that can and those that have to buy their score