If this was asked of me 4 years ago when I became a tmnt fan, I would've said FW as I thought Shredder and Splinter being humans who were once in the same dojo but now mortal enemies is the most fascinating (I absolutely love the rare moments when FW Splinter and Shredder fight and talk about their past).
However, I vote Nick now because it took the best idea and made it much better by adding the rivarly between the Hamato and Foot Clans, Karai, Tang Shen and so much more. Its one of the reasons why Nick Splinter is my absolute favourite incarnation of the character.
Edit: I like how Bayturtles is in quotation marks
Someone better not vote for it just to troll.