Voyager remains my favorite. Although I like all of the series. Any Star Trek is better than no Star Trek.
I never really got into DS9 first run thanks to the network that was showing it butchering the schedule. I missed huge chunks of the ongoing arcs thanks to sports preemptions all of the time.
TNG is good, despite the first two seasons looking bad from today's standards. The stories still hold true. ENT's first two seasons were mediocre at best, the last two were pretty sweet. TOS is TOS, it started it all.
But I have a soft spot for VOY. It was showing when I was going through high school and I really looked forward to watching it week after week. It was the Trek I grew up with and it aired at a time when spoilers were pretty much nonexistent for me. I did not know of any behind the scenes stuff. I just liked the characters, the ship, the premise, Seven of Nine...