In North America, Final Fantasy IV was the 4th mainline Final Fantasy game to get localized. Does that count

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I constantly flip flop between Megaman 7, 10 and 4 as my favorite in the classic series. 4 is absolutely my favorite NES entry tho even if I still love MM6 a ton. MM4 is one of my favorite looking games on that hardware, really enhanced the user experience with its redone menus for powerups and has one of the best arsenals in the franchise. After 3 games that had difficulty and pacing issues ranging from way too difficult to way too easy, MM4 also managed to find that sweet spot that even its followup - MM5 - wouldn't come close to hitting.
Resident Evil 4 certainly ends up that way for most people whether you are talking about the OG or remake and it isn't hard to see why.
Until Persona 5 brought the series to new heights of popularity, Persona 4 did the same back in the day and was considered by most to have been the best game in that series. That said its hard to count it cause most Persona fans you talk to have only played 1 or 2 games, so a lot of P4 only players back in the day would of course call it their favorite. Still, P4G was the most popular game for a while so I'll throw it on the list.
Interestingly, both Forza Motorsport 4 and and Gran Turismo 4 are considered the apex of their franchises by a good chunk of each franchise's fans. They both ended up being the apex of 2 respective console generations worth of improvements and additions to presentation, gameplay and featureset and were both followed up by a next-gen entry that ended up disappointing and cutting content whilst not refining the gameplay nearly enough to justify the generational jump in console performance. A bit sad how much they parellel each other despite being from seperate generations. I suppose you see the same with sports franchises. Jumping generations was a real rough time for most racing and sports games in the 7th and 8th generations.
I'll rapidfire off some other games that at least have some kind of push behind being considered the franchises' best:
- Pokemon Platinum (gen 4 even if it isn't the 4th Pokemon game cause that would be Firered and Leafgreen
- Teenage Mutant Ninja Turtles IV: Turtles in Time (I like hyperstone Heist more tho

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- Assassin's Creed IV: Black Flag
- Super Mario World
- Kirby Super Star (4th mainline game if you ignore the spinoffs)
- Fallout: New Vegas (it totally counts as a mainline release)
- The Elder Scrolls IV: Oblivion
- Uncharted 4
- Rayman Origins
- Civilization 4
- Command & Conquer: Red Alert 2
- Street Fighter III: Third Strike (counts if you think of the Alpha games as SF3 even if timeline wise it is a prequel)
Going into this I was skeptical of how many games I'd be able to think of but I'm shocked at how many exist. Depending on how you count games there are so many more too. I mentioned SF3 and Fallout: New Vegas already but if you only count numbered entries, GTA4 certainly gets a ton of love online too!