What I would like to know is the smallest bet with the biggest return but that's for another thread, in fact I will make one asking this right now.
So you're asking for the highest payout multiplier?
Just-Dice has a "one in a million" chance bet, which pays out 990,000x if you win.
I think 2 people won it when playing for BTC, and 2 more won it playing for CLAM.
See
https://just-dice.com/roll/929785969 and
https://just-dice.com/roll/941642049 for the CLAM ones.
Both players risked 0.0001 CLAM (about $0.0001) and won 90 CLAM (about $90). But I think they both took more than a million attempts to win, so they both ended up with a net loss anyway!