The most dramatic change in reputation I think is from Ninja Theory after Dmc, and then Ninja Theory after Hellblade ...

Rockstar with GTA3, Santa Monica with GOW and 989 with Syphon Filter comes to mind

Here are some games that helped the small time operations go big and get lots of attention.

Pong for Atari
Space Invaders for Taito
Galaxian for Namco
Donkey Kong for Nintendo
Pitfall for Activision
Dr. J vs Larry Bird for EA
Castlevania for Konami
Mega Man 2 for Capcom
Dragon Warrior (Quest) for Enix
Final Fantasy for Square
Adventure Island/Monster Boy for Hudson
Sonic the Hedgehog for Sega
Sim City for WIll Wright
Civilization for Sid Meier
Day of the Tentacle for LucasArts
Warcraft 2 for Blizzard
Donkey Kong Country for Rare
Tomb Raider for Eidos
Crash Bandicoot for Naughty Dog
Pokemon for Gamefreak
GTA3 for Rockstar
Shadow of the Colossus for Team ICO
Bejeweled for Popcap
Bungie for Halo
Infamous for Sucker Punch
Dark Souls for From Software
Minecraft for Mojang
Undertale for Toby Fox

What? Insomniac had a huge hit with spyro and they had a great rep due to it. 

I wouldn't go that far. There was some disappointment at the announcement of AC: Amiibo festival, but around the time it actually came out, Nintendo had Splatoon and Super Mario Maker going for it, which were pretty huge considering the size of the console base they were on. I'm not an AC fan so I didn't personally care as long as the other stuff came out. The Wii U as a whole was a big stumble for Nintendo, despite having some good games on it. :(

I'd honestly forgotten about Amiibo Festival, LOL.

lol I think people forget that Crash was a Playstation mascot waaaaay before Nathan Drake

Agree with Spyro being the game that made Imsomniac famous, it sold very well everywhere even in Japan

I get that Insomniac achieved great reputation before A Crack in Time. Hell, I thoroughly enjoyed Going Commando and Up Your Arsenal very much. So much that they're both on my Top 10 games of all time. But I feel that with A Crack in Time, they gained a higher reputation, a position they haven't been before. I'm assuming that due to this reputation, they've gone for Xbox for Sunset Overdrive years later. I'm not denying Spyro and the earlier R&C elevated Insomniac to higher levels, but ACiT felt like a huge step forward for the franchise and the developers.

Regarding Ubisoft and Assassin's Creed, I believe the franchise marked an before and after on the developer. Most of the games they develop have adopted certain aspects of its gameplay and made it part of them, for better or worse. Prince of Persia was definitely an excellent game, but I was pointing out which was, in my opinion, the games that changed their reputation in a good, moderate or bad way.

Rockstar- GTA

Not saying they had the best reputation, but it was a hell of a lot better than before the NMS launch. They were seen as this amazing small studio doing something amazing and with a decent track record from their bike game. Can't remember what it's called right now.