Highest?
Some 100-2000
BTC in a couple of online casinos.
Some 11k
BTC in Just-Dice. (a couple of years ago)
If we can include those who won by unfair means, Hufflepuff too, who made a couple of thousands from PD.
There is of course, the old SatoshiDice, which had some few thousand bitcoin wins.
Edit: That is certainly not my wins. Talking about "life-changing winners"
Edit2: Meet
https://www.moneypot.com/users/luthic Public Gambling Stats
Casino Bets Wagered Net Profit
Dust Dice 394 558,580,204 bits 38,764,819.94 bits
Roulett.oshi.XYZ 63 80,670,004 bits 7,780,001 bits
Around 45
BTC profit at the time of posting this.