I'm sure Dennis would say this show "hasn't even begun to peak yet", but I think it did a long time ago. I find season 3 and 5 to be the best of them with the highest number of amazing episodes and the least forgettable ones. Season 4 isn't too far behind. It must be mentioned though, that that is when I got into the show. By chance I caught an episode on TV, got all of the episodes and binged season 1-5 and then I've watched 6 to the current seasons as they've aired. While on my rewatches, I don't find season 7 to be as bad as I used to. I do think that's when the ideas started to fall away. There are some episodes in season 7 that don't do much for me. Much of the humour in 'Frank's Pretty Woman' and 'The Gang Goes to the Jersey Shore' is just shock humour - think the vomiting blood capsules scene - and 'Frank's Brother' is one of the worst episode from the series (competing with 'McPoyle vs. Ponderosa: The Trial of the Century' from season 11). 'How Mac Got Fat' is decent but is put together from an unaired episode. The final double episode of 'The High School Reunion' is a letdown as well. So, despite having some strong points like 'Thunder Gun Express' ("One of the things I like doing most is banging whores"), it has many more issues than what preceded it.
I must note though, that from what I've seen online, the above isn't a common opinion. Many love 'The Gang Goes to the Jersey Shore'. From there, I think season 8 was an improvement in consistency. They began to include many more call-backs, including the fun 'The Gang Recycles Their Trash' episode. Again though, the poor moments like 'Charlie Rules the World' are a lot worse than what we have from the earlier seasons. I won't review every season now, but the other issues on occasions have been the consistency of character. At times Mac in particular has been inconsistent with how he interacts with Dennis. From not listening to a word he says to acting like his puppy.
Many of the show's greatest episodes, like 'Mac Bangs Dennis' Mum, 'Kitten Mittens' and so on, have the gang break off into pairs or a triad and then come together for the episodes' conclusion. That structure seems to have become less common, as did the bar as the episodes' setting.
A little bit like The Simpsons, it isn't as consistent and amazing as it used to be, but for its peak, this show is a must watch.
Some of the best episodes:
Charlie Has Cancer
Mac Bangs Dennis' Mom
The Aluminum Monster vs. Fatty McGoo
Mac Is a Serial Killer
The Gang Hits the Road
Mac and Dennis Break Up
The Nightman Cometh
The D.E.N.N.I.S. System
The Gang Gives Frank an Intervention
The Gang Solves the Gas Crisis
Paddy's Pub: The Worst Bar in Philadelphia
The Gang Gets Whacked
Mac and Charlie Die
Paddy's Pub: Home of the Original Kitten Mittens
Thunder Gun Express
The Gang Tries Desperately to Win an Award
Season Rankings (need to add S12-14):
S5
S3
S4
S2
S6
S1
S8
S10
S7
S9
S11