Best Overall:
Fuzzy Dice: 2.5 XP/min, $10.00/min but highly intensive every 2 minutes
Best 30 Minutes:
Antenna Ball: 0.73 XP/min, $3.83/min (requires level 14...otherwise, "Oil Change")
Best 1 hour:
Neon: 0.45 XP/min, $3.52/min (requires Level 17, otherwise "Spark Plugs")
Best 2 hour:
Wash'n'Wax: 0.29 XP/min, $2.40/min
Best 4 hour:
Smog Check: 0.33 XP/min, $2.42/min (requires Level 15, otherwise "New Wheels")
Best 8 hour:
Monster Truck: 0.25 XP/min, $2.14/min (requires Level 23, otherwise "Car Detail")
best 12 hour:
DVD System: 0.20 XP/min, $1.99/min (requires Level 18, otherwise "Bullet Proofing")
Best 24 hour:
Nitrious Install: 0.14 XP/min, $1.68/min (requires Level 19. "Window Tint" and "Hydraulics" are exactly the same!)
Best 48 hour:
Suspension: 0.11 XP/min, $1.02/min (requires Level 16, otherwise "Engine Tune")
Best 72 hour:
Bodykit Install: 0.08 XP/min, $0.84/min (requires Level 20, otherwise "Biofuel Upgrade")