"Biggest drop in quality" sequels

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Kresnik said:

Spyro: Year of the Dragon -> Spyro: Enter the Dragonfly
Crash Bandicoot 3: Warped -> Crash Bandicoot: The Wrath of Cortex
Wip3out -> WipEout Fusion
Battle Arena Toshinden 3 -> Toshinden 4
Final Fantasy IX -> Final Fantasy X
SSX 3 -> SSX On Tour
Command & Conquer: Tiberian Sun -> Command & Conquer: Tiberium Wars
Super Smash Bros. Melee -> Super Smash Bros Brawl (COME AT ME, BROS!)

I've felt let down by a lot of sequels over the years, especially when the games that came before them are among my favourites of all time.

I believe the general consensus is that Melee is above Brawl.