I think I stopped at Black Flags, basically when they got rid of the number and the games stop feeling Assassins Creed, although I am missing AC and ACII but the editions were nothing to write home about, I started with the Brotherhood Codex edition.
The thing is when it got to Assassins Creed III, the collectors edition got samey, they followed the exact same structure.
Statue, Steelbook, Lithos, Soundtrack, Little Trinket
Any other time it's a good checklist to follow but when you have a franchise is comes out so often, it starts to feel reptitive and there's no weight behind them, especially when the prices seem to keep going up and up with each new instalment despite offering roughly the same things. The other issue being that in some regions you have different contents when really the top collectors edition, the most expensive one should include EVERYTHING from the previous editions, however because it's Ubisoft you'll have the middle limited edition for example offering you a small trinket not found in most expensive one.