What is the best bonus multiplier?

While I would have said the jet engine bike until recently, if it's XP you want multiplied, the biggest multipliers are the new Springfield Heights buildings. Placing all of them (even without upgrades) gets you a 150% multiplier for your XP! That dwarfs the puny 48% or so I'd manage to accrue up until now with regular bonus multiplier stuff (I'm a mostly freemium player).

But, if you want your cash multiplied, too, then yes, the JEB offers the best donuts-to-% ratio.