I think this totally depends on the generation you are from and the kinda person you are. For me I grew up on TNG re-runs & DS9 was being aired for first time. For others who grew up during the orginal series would say TOS is best and will always be the best for them. I love DS9 for it's epic arc story along with B5, they bring back memories
Generally that may be true, but it's not necessarily what generation you are from..
I was born in 87, grew up watching TNG, and for a long time it was the only trek i enjoyed. These days, I probably watch TOS more than any other version of trek. I find it to be the most fun to watch and that gives it more replay value for me. I think in a lot of ways it's the best show. I'm not saying it IS the best show though, because like i said before, it really depends on my mood, and I can't really decide which is the "best".