None. More people get to play certain games if games are multiplat, and the companies get more money from it. Everybody wins.

Sony
1. Metal Gear Solid the fanbase is on Playstation, and Sony was a major supporter of MSX computers which they also produced.
2. Final Fantasy
3. Kingdom Hearts

1. Conker
2. Megaman
3. Dragonquest

Microsoft
1. Mass Effect
2. Call Of Duty
3. BioShock

Edit: I know this was intended to be Nintendo and Sony only but i feel MS deserves to be in this conversation aswell.

From a perspective of 'buy this to dominate':

Nintendo:
- Final Fantasy (It is from the era of most of Nintendo's own series so it would benefit from being housed with comparable game, plus Nintendo could take the time to repair it's reputaton.)
- Megaman (Same, but with the alteration that it simply needs to come back rather than be reborn.)
- FIFA (Having FIFA, or Pro-Ev, as an exclusive would help them with their party-game scene.)

Sony
- CoD (Would totally undermine Xbox)
- Crash Bandicoot (Would give them a character with history and offer a stronger reward for a sense of fan loyalty)
- GTA (The series being linked to Sony would offer them an increasingly 'classic' ace-in-the hole that fits in with them perfectly.)

In terms of what it feels like they should be entitled to in the public gaming consciousness.

Nintendo
- Sonic
- Megaman
- Trauma Center

Sony
- Final Fantasy
- Crash Bandicoot
- Spyro

Nintendo:
Bayonetta
Megaman
Resident Evil

Sony:
Crash Bandicot
Kingdom Hearts
Final Fantasy

Nintendo
1. Dragon Quest
2. Harvest Moon
3. Final Fantasy

Sony
Elder Scrolls
Mass Effect
????

nintendo:
bravely default
monster hunter
sonic

sony:
tekken
resident evil
final fantasy

NINTENDO
- Sonic the Hedgehog
- Crash Bandicoot
- Spyro the Dragon
- Mega Man
- Banjo Kazooie
- Resident Evil
- Sly Cooper
- Ratchet & Clank
- Jak & Daxter
- Street Fighter
- Monster Hunter
- Kingdom Hearts
- Metal Gear Solid
- Pac Man

Sony:
Souls series (they started it with Demons Souls)
Metal Gear Solid
Persona or Crash Bandicoot

Nintendo
Killer Instinct
Banjo Kazooie
Perfect Dark

so basically all of the Rare games Nintendo lost.

Sony:

1. Crash Bandicoot
2. Metal Gear Solid
3. Resident Evil
4. Grand Theft Auto
5. Kingdom Hearts

Others: Devil May Cry, Final Fantasy, Tekken, Spyro, Souls series.

Nintendo:

1. Mega Man
2. Sonic
3. Banjo Kazooie
4. Bayonetta
5. Monster Hunter

Microsoft:

1. Mass Effect
2. Call of Duty
3. Splinter Cell

That's easy to answer. absolutely NONE. far better for everyone to not be getting screwed over by a single hardware vendor by them using a franchise to lock you in. I would much prefer if all 3 of the big names did less first party lockin.