I never watched the earlier series. I only recently got into TMNT from the 2012 show...and it is amazing. Even if I don't compare it to the other incarnations, it's a great stand-alone series.
Despite it being CGI, it's amazingly animated. Most if not all the characters look realistic and have great texture. And the dark tone is done very well.
The story is great too. It's surprisingly gripping and emotional, almost every character is 3 dimensional, and nothing feels forced. The action scenes are phenomenal, especially the ones with Shredder. Very intense and doesn't shy away from brutal violence.