This is what everybody seems to be missing in levels -that you need to be able to see it, straight on, in shooting position(just before pulling the trigger). That knocks most(by far) out of consideration. Doesn't matter how 'nice' they are.
IMO, the best, as it's most field functional, is the ScopLevel:
Scoplevel Anti Cant Leveling Device
You can see it straight on just before trigger pull, and it takes level to the actual scope instead of rings, base, action(which don't matter). That is, once it's set to the scope, you can move that scope from gun to safe to another gun & back, and the scope is always level when the bubble shows it. Portable in that sense.