Insomniac Games - They've mentioned numerous times that they love their independence, and have no want to be purchased. I'm sure Sony has put the possibility on the table multiple times, much like they did with Naughty Dog back in 2001, but the fact that they haven't been picked up yet makes it pretty unlikely it'll ever happen, barring Insomniac suddenly falling into a very weak situation financially.
Media Molecule - This is the studio I'd most love for Sony to buy (well, them and From Software), and for awhile I just assumed it would happen, given the success of LBP and the trend of Sony buying developers that they have a nice working relationship with. However, these days I'm not so sure. I've been reading more and more about Media Molecule's history, and the team leads left Lionhead solely because they favor the creative freedom that comes with independent development. I'm not so sure they'd be fine with jumping back into first party development anytime soon.
Quantic Dream - They're quite a bit more than just a development studio, and Sony only really works with them on Heavy Rain, afaik. Unlikely that this would happen, though I suppose the tech they work on could be useful for other first parties.
Sucker Punch - This is a possibility. but I also feel they're in a situation somewhat similar to Insomniac. If they were going to be bought, it would've happened long ago. I also don't think they're as productive as some other studios. Their games don't sell as well as even Insomniac's, yet they spend at least twice as long in development.
ThatGameCompany - I'm sure Sony has had some thoughts about buying these guys, but given how small scale their projects are, it probably wouldn't change the status quo much to do so. I think TGC are quite happy where they are, and they are also probably in the "indy studios making indy games are the best" camp, so they'd probably want to stay independent.
United Front Games - These guys actually seem like the most likely to be bought, but that's probably only because I know so little about them. They seem to be making a rather high quality game, and and don't seem to be on the "indy is cool!" side of the fence, so I could see Sony snatching them up if ModNation is successful and their next title turns out well.
In the end, I'd most want Sony to buy Media Molecule (lock up their amazing talent for the future - LittleBigPlanet is the best showcase for what the PSN/PS3 are capable of, and I'd love to see more of that from these guys) and From Software (with a larger budget and tech support from Sony's other first parties, Demon's Souls could become even better, and Armored Core could turn into the top tier mech franchise it should be - it's always been very rough around the edges).
I don't think either of these are very likely candidates, however. =/
*chooses Media Molecule anyway*
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