For me the original is the best. Part of it is the quality of storytelling, part of it's nostalgia.
I rate TOS and TNG as being about equal if we just think of TOS S1-S2 vs TNG S3-S4. I prefer TOS's stylised atmosphere, warmth, humour and generally stronger, more consistent writing. But I also prefer TNG's more serious, less campy, more science-fiction, more character driven and more conceptual approach. Love Picard and Kirk. Despise Troi and Sulu.
DS9 has the warmth and humour of TOS but with the more serious TNG approach there too. The best of both worlds.