Stick to the jumped shark.
The way I feel about any series is thus:
- I like loved Halo: CE. Halo 1 was kick ass, a great release title, and more importantly, I was fucking awesome at it. In an age just before the advent of online gaming, I was the king of the cafeteria tourneys. Story was good, within reason, and made logical sensicals. No over the top villany or heroism.
Halo 2 I liked. It was a bit more over the top, and I really disliked them taking away some of my favorite parts of H:CE, such as stunlocking plasma rifles, the power of the plasma pistol, pistol scope, and assault rifle, but overall it was a bit more balanced if not easier in SP. The story was about standard fare for a sci-fi game, but made enough sense and had enough feel to get me to appreciate it.
Halo 3 I .... played, but by then I was sick of the series. Flood has new forms, everything is so fucking over-the-top, and the escalation of power has moved to the point where why do I even bother playing? Theyre just going to hold my balls while I pee anyway. Story was less adequate; merely mediocre, and the powerups have become literal invincibility for you. Fucking mario game.
Other series do this too, but I really tend to drift away from a franchise as it matures from 'exactly the level of plot I want' and usually good mechanics that make the player appreciate his limitations to 'fukheug power' and feeling like superman.
Look at Doom3 compared to the newset doom; thats one example. I liked D3. I didnt think it was the best game ever in the universe, but I liked its challenge for what it put out. I liked feeling limited and pitted against a vast horde of deadly enemies, and I like how they did nightmare mode. In superhard mode your health was drained until it hit a max of 25. Essentially 75% of your max health was replaced by overcharge health. In D3:2 they fucked it up by making you face increasingly escalating difficulty and turned it into a bit of a grind, but in the sequels they made it even worse by removing the limitations of the new DoomGuy and making him more like SPACE RAMBO.
Jumping the Shark is when the world gets 'surreal, even for fiction and sci-fi' and when the protags are so noble they fart helium.