I wouldn't go back to the PS3 versions of games which have a better PS4 version.

(Last of Us Remastered, DMC, Sleeping Dogs, Beyond Two Souls, Uncharted 1 - 3, Heavy Rain, God of War 3, Journey and many others).

These are the games I keep my PS3 for:

• LittleBig Planet 1 + 2
• ICO / SotC HD Collection
• God of War 1 + 2 HD Collection
• Okami HD
• Wipeout HD / Fury
• Demon's Souls
• Metal Gear Solid HD Collection
• Metal Gear Solid 4
• Ratchet & Clank HD Trilogy
• Ratchet & Clank Future games
• Sly HD Trilogy
• Sly 4
• Jak & Daxter HD Trilogy
• Killzone 2 + 3
• inFamous 1 + 2
• Ni No Kuni
• Resistance 1 + 2
• Heavenly Sword

Some of the best PS3 games have PS4 remasters but ill list them anyways: (Assuming you're asking for PS3 exclusives only)

- The Last of Us
- Uncharted 1-3
- God of War HD Collection/Origins Collection/3/Ascension
- Ni No Kuni
- 3D Dot Game Heroes
- Catherine
- Yakuza 3-5
- Ico/SotC HD Collection
- MGS HD Collection (I know its not exclusive but wanted to add it anyways)/4
- Kingdom Hearts 1.5/2.5
- Demon's Souls
- Killzone 2/3
- Infamous 1/2
- Resistance 1-3
- Little Big Planet 1-3

Ratchet & Clank Future tetrology (Tools of Destruction, Quest for Booty, A Crack in Time and Into the Nexus).
Ratchet & Clank HD Collection
Sly Cooper HD Collection
Sly Cooper 4

Valkyria Chronicles (HD version is available for PS4)
Heavenly Sword
Nier, Resonance of Fate, Eternal Sonata (the three of them also available for Xbox 360)

Those and Xcom: Enemy Unknown are the games available for PS3 that I enjoyed.

Ni No Kuni and 3D Dot Game Heroes are good