I used to use different themes, now I have used one theme for the last 10 sites I built. Its the Striking theme from Themeforest. Its super flexible, lots of options, and tons of solid support!
Hard to just single out just one great theme. I usually go for premium themes and do some own personal customizations. I like the themes at theme forrest and yoothemes.
The best theme to use is one that is unique to your site and which no other site is using - that way your site will look unique as well as having unique content.
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