Big
It is no good chasing down big bonuses if the casinos don't pay when you win.
A couple of the others in the same group as challenge have pretty big bonuses.
The 30% bonuses at Roxy and Casino Splendido have a habit of being turned off if you actually take them up too often. I had both mine turned off before I even managed to reach Platinum status.
Jackpot Factory have a decent regime for VIP's, and it does not seem to turn off so readily. As well as 300 credits per week on a 2000 credit Tuesday deposit, they run a cashback of up to 300 credits on Saturday and many promotions that add extra chips. There are also scoreboard promotions with substantial prizes based on wagering and big payouts. The current one has a weekly top prize of 2500 credits, and VIP's will only have to wager this 15x.
Phoenician has retained it's 1000+ signup offer, but is now a Casino Rewards property, and their CS leaves a lot to be desired, and withdrawals are limited to 4000 credits a week.
Some dodgy Playtech casinos offer large bonuses, but getting paid can be a problem as they are likely to label winners as "bonus abusers" and just refund the deposit.
The biggest bonuses of all have come from the RTG stable, but so have the dodgiest operators, so research is essential (Avoid doing the 15000 credit bonuses at Crystal Palace group - getting paid is going to be a problem, they will find some excuse just to refund the deposit.