I wonder if ditto could be the best defender if the big-hp-mons are nerfed
Almost no chance of this happening. For defenders, bulk is the best quality, ability to deal damage to the attacker is the second best quality, Ditto can't really guarantee either of these. A lot of pokemon are going to be bulkier than Ditto no matter how they do the rebalance.
Ditto will never be a good defender with current transform mechanics. The moment the attacker realizes he can force it to transform into a rattata and proceed to beat it up with his machamp it's over
Due to how transform works (keeps hp, copies other stats) ditto is doomed to be strictly worse than anything over 96 stamina. So it can only be better than some unevolved pokemon + kadabra, haunter and kakuna. It's sad