What is the minimum level I need to be to get the strongest version of leveled items?

UESP says 46 is the highest level required for leveled items. Level 48 is where you can start finding Daedric armor. We might need the Creation Kit to be sure about every single item.

In Oblivion it depended on the item. The level variations were part of the item's game data, so each item would have custom level breaks. It's a very useful feature for the designers to have, so we have every reason to believe it's the same in Skyrim.

Until we have access to the CK, or until someone extracts the item data with a custom tool, we won't be able to say with certainty what character level would trigger the highest tier of every levelled item in the game.