Which is better? Star Trek TOS or TNG?

As others have said, both had some great episodes and both had some that were... not very good.

In both cases, there were reasons, as TOS was made for relatively little money and with very tight production time margins. TNG had more of both, but frankly, by 1987, Roddenberry had bought into his own myths, and he was never a good writer himself.

I'd have to vote 51/49 for TOS and that includes the early 70s Animated Series.

It's hard to choose. As much as I really like both equally, but I gotta go with Star Trek: The Original Series.

While Star Trek TOS & TNG had better actors and plots I still have a soft spot for the original Star Trek. "The Trouble with Tribbles" is still the best episode. 

The Next Generation was better.

Original is the best

 Star Trek TNG.

In my opinion,  Star Trek TOS 

Both were great in their own way.

 neither. both are garbage 

As good as future spin offs were........you can't beat The Original Series.