Based on their recent work, success and budget -
S Tier: Naughty Dog , Santa Monica Studios, Polyphony Digital, Insomniac Games, Sucker Punch Productions.
A Tier Guerilla Games (bordering on A/S but Horizon 2 might put them on S), Housemarque , Team Asobi, Media Molecule.
B Tier : Bend Studios , London Studios, Pixelopus, San Diego Studios.