Flatline OPS, hands down in my experience is the best on the market. They have models for 1", 30mm and 34mm scope tubes, they fold out of the way and yet are still useable in that position, and best of all, they can be calibrated. I disliked my US optics level, but if you don't want to spend the money on the flatline ops then get a cheap scope tube level from vortex or sinclair intl.
Believe me, if you can afford the flatline ops, you won't be sorry.