At what level(s) would the main quest be the most unique, yield the best rewards, and present the best experience?

I would say it's less a matter of what level and more a matter of how much fire resistance you can acquire. Alduin's fire breath huuuuuurrrrrts.

As for levels, I don't know. But I usually start once I have steel stuff.MQ is good for all levels.

I did it once and it was pretty basic.

At a higher level it had better loot, different enemies, etc.

But I reckon doing Companions is good enough, I am level 11.

Level 50 and above. That way, you'll face Deathlords and Priests. You get better loot and you'll overall feel more accomplished.