What? How do they have a weak first party? They have Ensemble, who made the Age of Empires series. They have Bungie who made Marathon, Myth, and Halo. They have the teams behind Project Gotham and Forza Motorsport. They have Rare. They have Lionhead who made Fable, a fantastic game. Weak? What have you been smoking?
So yes, it can be argued. It can be argued quite well. Sony's first party is the weakest, and most overrated of the three. They rely on their third party support for most of their big games.
Bungie is unquestionably a feather in their cap., If the best you can mention about Ensemble is that they've made Age of Empires (not a renowned RTS series but a very good one I'll give you that) then you'd best not mention them. Project Gotham is a good dev, but not a standout, the Forza guys deserve props for sure. Rare is a shadow of its former self, PDZ was bad, Kameo was underwhelming, and Viva Piniata is a good game. Congrats it only took them pissing on one of the best IPs of the N64 generation to acomplish an actual good game for Microsoft. Lionhead has made a good game in like... a long time and we'll leave it at that.
FASA has been shut down FYI.
Sony has
Guerilla Games - Killzone, Killzone: Liberation (one of the best games on the PSP a standout product), Killzone 2
Polyphony Digtial - Omega Boost. Gran Turismo, Gran Turismo 2, Gran Turismo 3: A-Spec, Tourist Trophy, Gran Turismo 4, Gran Turismo 5
Naughty Dog - Crash Bandicoot, Crash Bandicoot 2, Crash Team Racing, Jak and Daxter, Jak II, Jak III, Jak X Racing, Uncharted: Drakes Fortune
Insomniac - Spyro series, Ratchet and Clank, Resistance
Sucker Punch -Sly Cooper, Sly Cooper II, Sly Couper III, Infamous
TEAM ICO - ICO, SotC
Clap Hanz - Hot Shotz series
SCE BEND - Syphon Filter
Santa Monica/Incog - Twisted Metal, Twisted Metal II, Twisted Metal Black, Kenetica, God of War, God of War II, God of War III
Zipper Interactive - Soccom
SCE London Studio - Singstar
SCE Studio Cambridge- MediEvil series, Primal
SCE Studio Liverpool - Wipeout series, F1 series
Ready at Dawn Studios - Daxter, God of War: Chains of Olympus
SOE - Recent intergration.
In term of overall strength and diversity SCEWWS is clearly a bigger and broader entity, producing good games for every segment of the population demographic. Sure they don't have an FPS to match Halo yet, aside from Resistance which is arguable, but they have quality products for a much broader range, coupled with having the best platforming dev in the industry, one of the best racing devs bar none who is certainly above the PGR team and will soon unseat the Forza Team again (Forza team gets my nod for overall innovation, but GT5 is on the way and they are clearly a more adept studio then Team Forza). SOny clearly has better overall first party studios that deliver quality products to amuch broader range of gamers then Microsofts first party.