I don't think it's possible to definitively quantify the term "best." My list would be so heavily tainted by nostalgia and personal taste as to render it irrelevant outside my own head. Many shows other people gush over leave me cold, and some shows that started out great ended on such sour notes that they tainted the whole works (*ahem*X-Files*ahem*.) TBH, I stopped watching new shows a while back because I got sick of the good ones being axed or irredeemably ruined.
That said, if you insist (subject to change, and in no particular order):
Buffy the Vampire Slayer - The only TV series that I've seen nearly every episode of and only really didn't like one. Plus the finale was one of the greatest wrap-ups I've seen on a series.
Police Squad! and Sledge Hammer! - Two shows that were ahead of their time; they would've done much better had cable been a viable option.
The Wild, Wild West - Nostalgia talking, here (didn't see the original run, but saw it in reruns), but aside from a shaky final year they hold up pretty well.
The Avengers - The original British series, not the superhero franchise. They lost a little steam later on, but they were great fun and wildly imaginative.
If partials are allowed: the first four or five seasons of The X-Files, the first four seasons of Law & Order: Criminal Intent, the earlier Scrubs, and the early original CSI. I also liked what I saw of the rebooted Doctor Who, but due to family conflicts I had to quit watching. (It's no fun watching a show when relatives whine and moan about the thing incessantly.)
I also give an honorable mention to The Equalizer. Why? Back when I was in grade school, my sister was a huge Hunter fan - and I usually wound up watching it with her. One night, she tuned in early and we caught the last 10-odd minutes of The Equalizer, where a man was talking about his near-death experience in Hell and it was implied that the star had had a similar experience himself. To this day, I can remember almost nothing about Hunter (aside from the lead guy being about two feet taller than any woman he worked with), but I can remember that bit of The Equalizer. (Later watched the full run on A&E, and enjoyed most of it, save that last part after the star had had a stroke.) Any show that can be remembered more than twenty years later, on the basis of ten minutes, has to count as good.
