Witcher 3 was peak
Warcraft 3 was peak
Diablo 3 was peak (sue me)
Kingdom Hearts 3 was peak (sue me more)
DMC 3 was peak

So I'm gonna have to say third game though obviously there are examples to the contrary.

For Final Fantasy, the first four generational trilogies peaked at the third game and had a dip in the middle.

Looking at duologies / trilogies (so ignoring entries outside the OG 3) for me the peaks would be Halo 1, Sonic 3 & Knuckles, Gears of War 2, Fable 2, Resident Evil 2, Borderlands 2, Red Dead Redemption 2.

That's a tough one. I can think of examples for 1st, 2nd, and 3rd:

Peaked at 1st game — F.E.A.R., BioShock, Dead Space, Metroid Prime, Tomb Raider (Core Design), Batman: Arkham, Infamous, Halo, Jak and Daxter, Xenoblade, Ninja Gaiden (Black/Sigma)

Peaked at 2nd game — Uncharted, Fable, Bayonetta, Tomb Raider (Survivor trilogy), Resident Evil, TimeSplitters, Zero Escape, Doom (modern trilogy)

Peaked at 3rd game — Darkstalkers, Street Fighter Alpha, God of War, Gears of War, Metal Gear Solid, Ratchet & Clank, Super Mario Bros., Darius

Going in, I guessed the third game would be the winner, but I'm not sure now. Also, I'm certainly overlooking some trilogies (and unsure if the listed ones all count). I suppose if I had to choose, I would pick the first game.

FTFY.

Nah, I said what I said, and yes, that goes for FF12 versus FF10 as well.

I was giving you the benefit of the doubt that FFX-2 was the dip in the middle and not FF11.

Games hardly peak on their first entry if they become a franchise, I can't think about many definitive examples.

TLoU1 is perceived as clearly the best online, but I personally don't think so, TLoU2 is at the same level and I consider it higher than 1 nowdays.

Best games in some franchises:

Sonic: 2
Spyro: 2
Crash: 3
God of War: Ragnarok
The Last of Us: 2
Uncharted: 4
Gran Turismo: 7
Ace Combat: 2
Tekken: 3
Resident Evil: 4
Metal Gear Solid: 3
Final Fantasy: 12/9
Persona: 3
Disgaea: 2
Zelda 2D: Link's Awakening
Mario 2D: World/3
Pokemon: Crystal (gen 2)
Digimon: World 3
Age of Empires: 2

Horizon Zero Dawn is an example, Forbidden West is better as a game in every way but Zero Dawn story is just so unmatched I can't rank 2 over it.
Ghost of Tsushima pretty much the same scenario, Yotei is better in everything as a game but Tsushima story is better enough to rank it higher to me.

Diablo 2 is widely considered the best game in the series but for me 1 is the best.
Marvel vs Capcom is pretty much the same too, 2 is considered the best by nearly everyone, I like 1 more.

If we consider Souls as a whole as a series, then yes, Demon's Souls in the best. Dark Souls 2 has grown on me more than 1 if we consider only the Dark Souls actually franchise tho.

The pattern is that games usually peak at their 2nd or 3rd game for me, with a few franchises having the newest games if they are incremental like Gran Turismo, Ace Combat (even as I pick 2, 7 is the best to play today), Tekken (same, I pick 3, but 8 is the best to play today).

The franchise peaking in the first game is very rare, both for me and for most people online too, as I pointed, even some of the ones I picked the 1st myself most people online pick the 2nd as the best

Out of this world was also my favorite out of this world game and there is only 1 game in the franchise so there is another argument for 1