Off the top of my head:
one of the biggest nominal bonus is one of the Rewards casinos at $800 probably Aztec Riches. Then there is 21 magic or magic box (always confuse the two) with $1000 and I think Collosseum also had a big bonus but that may have changed. And millionaires club had a big match bonus.
With regards to biggest relative bonus I think All Jackpots and All Slots used to be in the region of 150% to 200% i.e. deposit $25 get $100.