It is not a simple answer to what the best free spins bonus is but there are a couple of factors that affect how good a free spins bonus is. Follow these steps and you will evaluate free spins bonuses as a professional.
Winning Cap: Most free spins bonuses have a cap on winnings. This cap shall obviously be as high as possible.
Wagering Requirement: Casinos always put some terms and conditions on a free spins bonus. The wagering requirement shall be as low as possible, preferably zero.
Betting Limits: You obviously want the highest betting limit possible. Unfortunately most free spins are tied to the minimum bet.
Amount: The more free spins the better would most people answer but the steps above are far more important. 5 spins with no winning cap or wagering requirement have far better potential than 100 free spins with a winning cap of $10 (or equivalent).