I think you should probably avoid the Playbook, it doesn't have great support for apps, and Android will be easier to side-load media onto.
Between the Galaxy Tab 7.0 and the Nexus 7.
Plus for the Nexus:
- Faster processor
- Higher res screen (216 PPI)
- Stock Android with no skins
- NFC
- Fast updates from Google (Sometimes only a couple of days)
- Better viewing angles (IPS screen)
- Front Camera
Plus for the Galaxy:
- Expandable storage
- Haptic feedback
- Rear Camera
I've got the Nexus 7 and love it.