Watch seasons 1 through 4, but in season 4 stop at 40:47 into the last episode (4×22 World’s End) just after she says “Anyone have room for some pie?”
That’s it. Just stop there. That’s a good ending to the series.
After that it’s basically a season of bottle episodes in Season 5, on a new set they built (somewhat similar to Stargate SG-1 season 8, although in that case they were reusing their set).
And not only is season 5 all-bottle, all the time, but its a horrible dystopian future that just goes on and on.
The only episode of season 5 that has any spark at all is 5×05 Rewind, with the reappearance of Hunter bringing both some much needed energy and humour as well as leading to them actually going outside.
Marvel’s Agents of SHIELD season 5: Dystopian Future
Seriously why would you want to watch hours of characters you like stuck in a grindingly depressing and intermittently horrifying and disturbing dystopian future.
Season 6 the team isn’t even together, plus which they still don’t get out of the set very much (the same set from season 5), plus which its clear the writers have kind of given up and are just writing stuff to amuse themselves or try out new things for the characters.
Marvel’s Agents of SHIELD season 6: Lurking Underground or Wandering Around In Space While Earth is Beset by Killer Birds
The only redeeming feature of seasons 5 and 6 is Enoch (Joel Stoffer) but he is not enough to rescue either season.
Season 5 had 22 episodes (and a rather unsatisfying resolution). Season 6 has been mercifully cut to 13 episodes, as presumably will be the announced final season 7.