Eldritch blast with Hexblade's Curse and Agonizing Blast.
And it's not even close. A warlock that can cast eldritch blast twice in one turn is the strongest character. At 5th level with the Agonizing Blast invocation, that is four attacks dealing 1d10+CHA each per turn. At 11th level it becomes six attacks, and at 17th it becomes eight. But wait there's more! Picking up the Hexblade patron grants access to Hexblade's Curse, which allows you to add your proficiency bonus to the damage too. A 17th level warlock can deal 1d10+11 damage per hit (with 20 Charisma), for eight total attacks in one turn.
In this answer I outline the alarming power of being able to cast eldritch blast twice in a turn. Without Hexblade's Curse, the average damage against an AC 18 target works out to 82 damage per turn; whereas adding Hexblade's Curse on top of it adds another 40 or so per turn. Granted, this is in the context of a 20th level character, but it scales down as expected. The number of attacks, the strong damage die, and the ability to add charisma and proficiency bonus to each hit makes eldritch blast far and away the strongest application of this rule.
But you're a wizard! If multiclassing is allowed, all you need is two levels of Hexblade Warlock to unlock Agonizing Blast, and your eldritch blast beams scale with your total character level, not just your levels in warlock.
"All casters get a very rare magic item for free" is pretty unbalanced.
This rule is actually already a magic item in the game, the very rare Illusionist's Bracers:
While wearing the bracers, whenever you cast a cantrip, you can use a bonus action on the same turn to cast that cantrip a second time.
As a very rare item, its value is somewhere between 5,000 and 50,000 gp, but I've used this item before: it is one of the most powerful magic items in the entire game. I would put this on the 50,000 gp end of that spectrum. So your DM has essentially given every caster one of the most powerful items in the entire game, for free, without attunement restrictions.
To be clear, this is not "your fun is wrong", if every one is having fun. But be advised, this does make all casters way stronger than all martials. Cantrip damage scales up for casters to match the increase in damage output of martials that get extra attack. If I were in this game playing a fighter, barbarian, or monk, I would ask to switch classes, because there is no point in me doing anything anymore because the casters are going to be overwhelmingly powerful in comparison.