It's a close run thing for me between :TNG and DS9.
:TNG on the plus side was the idealism, the cast, especially Patrick Stewart (Why he hasn't been knighted yet is a mystery to me. If you ever get the opportunity to see him on stage you'll know just what a great actor he is ) and for me it was the series that hooked me in as a fan.
On the minus side, they didn't actually do a lot of going where no one has gone before. A great majority of episodes revolved around the enterprise going from one known planet to another known planet to provide diplomatic services/medical help, while the sub-plot was usually Data exploring some element of the human condition. As Picard's archaeology professor said to him "You're just like a Roman centurion, patrolling a bloated empire."
I totally agree with what W.I. said about DS9 about the balance of idealism and reality. I think that they dealt with issues better than :TNG e.g. The terrorism v freedom fighter debate. Also in later seasons we had a lot of Klingon orientated episodes and that's always a plus in my book.
It took me a while to get into Voyager. For the first few seasons I had this theory that they were the B Ark of Star Fleet and the Marquis. I will explain: For those unfamiliar with Douglas Adam's Hitch Hikers Guide to the Galaxy, the people of the planet Golgafrincham decided to get rid of their useless third of their population (hair dressers, middle management, telephone sanitizers etc.) by making up as story about impending planetary doom. They packed off the useless third onto one giant spaceship called the B Ark.
And that's what I thought Star Fleet and the Marquis had done with their useless personnel. They couldn't sack them, (the unions wouldn't have it ) so they packed them off to the bad lands and off to the Delta quadrant to keep them out of harm's way, so that they could get on with their nice little war with the Dominion.
Many an episode I'd have to watch them work out the blatantly obvious while shouting at the TV "It's an effing temporal flux", only to have to sit through 44 minutes until they work out that it was a temporal flux.
But that aside, I actually ended up really liking Voyager.
I won't go into the original series or Enterprise, because one, I have spent too long waffling on, and two, I've posted views on those two shows in the other Trek thread.
As for the films:
First Contact is my favourite, closely followed by The Undiscovered Country. Then it would be a tie between Wrath of Khan and The Voyage Home, with the former only just coming out in front.