This is tough, and once again i doubt i could pick a definitive 5...but 5 shows that would certainly make the cut are...
1.) Lost - To be honest, i think this would be #1 even on a definitive list, because it truly was my favorite show of all time...i wasn't a huge fan of the end, but the ride to that point was incredible...shows like this simply don't come around often enough at all.
2.) Firefly - This show was only 1 season long (with a motion picture "finale" so to speak), but man, i was emotionally invested in it, and was devastated when i found out it was cancelled so quickly. I love sci-fi, and have watched nearly every sci-fi series you could think of, but this one stands out for it's unique blend of westerns and space opera. I miss it.
3.) 24 - Me and my wife were glued to the set every week watching Jack Bauer do the same thing week in and week out...GREAT show that might have worn out it's welcome with too many seasons, but I still love it, and am very excited to hear a movie is in production right now!
4.) The Walking Dead - Putting aside the fact that this show is filmed in Georgia (which is awesome!), the story is breath taking with so much depth in both characters and plot. I've been an avid fan of the comic books for a while, and knowing what is coming (and the differences they add to the TV series to keep it fresh) is very exciting.
5.) Futurama - I grew up a Simpsons fan, it WAS my childhood...so when i became an adult and Futurama was released, i was instantly hooked. When they decided to make new episodes a few years ago, i was so stoked!