For eds with sweet spots, it's easy to pick. 1e, it was 3rd-7th (even though the game held together longer, that always seemed like the best bit to me). 3e, 1st-6th, maybe longer with sufficient player restraint.
4e, though, works at 1st, at 11th, at 26th, it doesn't matter. Each tier is different, some campaigns should probably focus on one tier, for instance, because their tone/scope are best-suited to that tier.
OK, here's an answer: My favorite Tier is Paragon - when all the characters have been played up from first. The characters are established, the players know how they work and how to make them work together, but they've just picked up new tricks with their Paragon Paths, and the scope of the campaign shifts from Adventuring to more earth-shaking events.
I really would have liked an Epic Tier Handbook (DMG3) with advice to run campaigns that are not just D&D with bigger numbers.
Yes, like the DMG2 did with Paragon.