You, sir, rock. I don’t know whether to slap myself for not thinking of that myself :smack: or to beat you senseless with a club and steal your idea. 
Ooh… a better answer: With a billion dollars to invest in any one TV show, my money would go towards letting Joss Whedon do absolutely anything he damn well wants to. Flying monkeys? Robot apocalypse? Turducken? You bet, Mr. Whedon!
Can I recommend a live action “Dirty Pair” with the following:
Angel (but unlike silenus, because I didn’t care for the ending)
Firefly
Wonderfalls
Freaks and Geeks
The Electric Company
And, well, Buffy. But only if they made it as good as the 2nd, 3rd and 5th seasons.
El Diablo Robotico!
What, no love for MacGyver?
Or does that count as trolling?
I’d have to say Bonanza, because in addition to being rich, to pull this one off, I would also have to have god-like power over life and death.
Several of my favorites have already been mentioned: The Adventures of Brisco County Jr., Wonderfalls, Futurama, Carnivàle.
I’d love to see new episodes of Reboot, Dinosaurs, Weird Science, Eerie, Indiana, and Duckman.
Joan of Arcadia.
There was an extremely short-lived show on, IIRC, ABC a few years ago that was set in a mental hospital. Lasted all of two, maybe three episodes. Very gritty stuff, and I loved it, but apparently it offended the crazy folks. I’d like to give that another shot.
Junkyard Wars
Definitely Freaks and Geeks.
Goodbye.
Oh, good one. As long as it veers back away from the soap opera that was Bailiss toward the end.
I didn’t know it had been cancelled.
I agree with others on Adventures of Brisco County Junior & Futurama. However seeing as how I am completely irresponsible with my tens of dollars, I have to say I would also be irresponsible with billions of dollars. So I would also bring back…
Wonder Woman
The Green Hornet
Hello Larry
Real People
Automan
Mannimal
Werewolf
Add me to this bloc.
I didn’t think anybody remembered the TV version of that! (Even though, according to IMDB, it ran for five seasons).
Somehow I doubt watching it now would live up to my pubescent fantasies of Lisa (Vanessa Angel, so much better than Kelly LeBrock, the movie Lisa).
Babylon Five
Space Above and Beyond.
Hill Street Blues
NYPD Blue
Seinfeld
Frasier
Barney Miller
The Bob Newhart Show
The Mary Tyler Moore Show
oh and…Twilight Zone
Cupid.
And I second Joan of Arcadia.