My first thought is to say All in the Family. This show changed TV. Before, everything was like Pleasantville: bland, white-bread plots, safe problems, laugh track for the comedies. All in the Family made TV real. Every show on TV today which deals with real problems in a real manner with real language can trace its history back to AITF, which was also an extremely well-written show.
I'd also wanna mention the original Star Trek. Also ground-breaking: the first adult-level scifi show with continuing characters (and well-written ones, at that!) that gave a realistic depiction of what the future could look like if we all worked together. Because it was scifi (and pre-AITF), it could also deal with real problems (eg, racism) cuz it could broach them "disguised," which has always been one of scifi's strengths.
MASH rates. Excellently written comedy against the backdrop of humanity's darkest activity: War. Very tough balancing act.
Simpsons: certainly the best animated show ever. Ground-breaking in that category.
I'm not sure if any shows since the 80s would fall into "Best Ever" ('cept for Simpsons). The 80s made greed fashionable, the 90s were the Party Decade, and since 2000-2001, $$$$ has become more God than ever, thanks to the current administration and its catering to the (his words) "haves and have-mores." Not that there arent any good shows now, but everything is geared, more than ever before, to being marketed and turning a profit, not just in TV but in movies and music as well.
...of course, this is all just my opinion. bag