Seasons 1-6: outstanding. Deep world-building, intelligent dialogue, epic set-pieces worthy of Hollywood, expert acting, detailed plotting, genuine surprises, great soundtrack used sparingly. GoT is worth watching for these seasons, definitely.
Season 7: you suddenly notice the writing and plotting has taken a sharp dive downwards in quality. There are inexplicable ideas, inconsistent characterisation and soap-opera dramas. But at least the entertainment value is still very high, so it gets a pass.
Season 8: this drop chasms down to depths you didn't think possible. The writing has got so amateurish that the still-fantastic production values are no longer enough to save it. If you ever enjoyed Lost but were amazed at how bad the final season was, you're in familiar territory here.
The biggest problem with the final season was it took us out of the story and got us thinking about its many weaknesses. We lost our suspension of disbelief. Fatal for fiction. It also ruined the series-arcs to the point where a rewatch of the entire series is pointless.
So can I recommend a GoT marathon for newcomers? Yes, the first 6 seasons are worth the disappointments later on. But best series of all time it is not. It could've been, but dropping the ball in the final stretch puts it behind the following superior series more worthy of your marathon:
- Battlestar Galactica (2003-2008)
- Twin Peaks (all 3 seasons, plus the movie)
- Breaking Bad