The following are all required to obtain 100% completion:

Story Missions
Complete all main Los Santos missions (26)
Complete all main Badlands countryside missions (14)
Complete all main San Fierro missions (20)
Complete all main Desert missions (8)
Complete all main Las Venturas missions (17)
Complete all final missions (9)

Side Asset Missions
Courier (Level 4 Completion)
Courier (Level 4 Completion)
Courier (Level 4 Completion)
Quarry (Level 7 Completeion)
Trucking (Level 8 Completion
Valet Parking (Level 5 Completion)

R3 Missions
Taxi Driver (50 Passenger Fares)
Vigilante (Level 12 Complete)
Paramedic (Level 12 Complete)
Firefighter (Level 12 Complete)
Home Invasions ($10,000 Stolen)
Pimpin (Level 10 Complete)

Achievements
Spray all 100 tags in Los Santos
Collect all 50 underwater Oysters in San Andreas
Collect all 50 hidden Horseshoes in Las Venturas
Take all 50 San Fierro Snapshots

Gyms
Learn Boxing Style in Los Santos Gym
Learn Martial Arts Style in San Fierro's Gym
Learn Streetfighting Style in Las Venturas' Gym

Race Tournaments
Win all Los Santos tournament Races
Win all San Fierro tournament Races
Win all Las Venturas tournament Races
Win all Las Venturas air tournament Races

Vehicle Schools
Get Bronze or Higher in Driving School
Get Bronze or Higher in Motorcycle School
Get Bronze or Higher in Boat School
Get Bronze or Higher in Flight School

Ammu-nation Shooting Ranges
Complete Pistol Challenge
Complete SMG Challenge
Complete Shotgun Challenge
Complete AK-47 Challenge

Vehicle Challenges
BMX Challenge Completed
Chilliad Challenge Completed
Fright Train Completed - Railway Lines
Lowrider Challenge Completed
NRG-500 Challenge Completed
Races and Stadium Events (4)

Exports and Imports
Import List 1 Completed
Import List 2 Completed
Import List 3 Completed

Assets and Properties
Purchase and Acquire Zero's RC Toy Store
Purchase and Acquire Wang Cars
Purchase and Acquire Airstrip
Purchase All Safe Houses & Hotel Suites (33)