Rolling Thunder

Most Flexible when dealing with multi targets. Don't get me wrong I love fireball but sometimes deviding damage evenly rounded down while paying 1 for each target can be real bothersome

This you can devide it however you want to as many targets as you can... I'll take this over fireball mostly any day.

For total damage, Fireball.

To wipe out a lot of creatures, tokens especially, Dwarven Catapult.

For killing lots of creatures or any size, this.

This is the most utilitarian X point burn ever printed in the history of magic. For fireball you are basically spending X times the number of creatures you want to kill where X is the highest toughness. So if you wanna kill a serra, and two 1/1s you are spending R + 3 x 4 + 2 = 15 converted mana. With rolling thunder you are spending RR+4+1+1 = 8 converted mana. You be the judge of which spell is superior in nearly every single situation. Lavalanch MIGHT be better in jund, but it is pricey given its RBG initial cost.

basically replaced fireball in my elf burn deck. pay only one more red mana or pay 1 for each additional target, hmm which one would cost less?

A good burn card that has many uses. Great against swarm decks.