2k3 Raph and Mike after Seasons 2 were generally the weakest incarnations. Shame because they started off so well. After that Raph was always mad, Mikey was always loud or obnoxious, even when nothing was happening in an episode. It's even more odd now when rewatching the show years later. I don't want to watch a Raphael who is always angry 24/7.
Nick's Splinter is easily the best, one of the greatest incarnations of Splinter in general.
For April, again this is difficult. 2k3 April had a decent personality but she just kinda faded to the sidelines as the series progressed. OT April was always there but a damsel in distress, I liked Nick's April's personality wise.
As for Leatherhead, 2k3/Nick Leatherhead are practically the same exact character.
The Shredder's all worked well for their respective series so I can't rank them. They all did a good job for the tone of the series they had.