is Magneto/Zero/Dante the best rushdown team in the game? If you get a hit with Mags or Zero its going to lead to lightning loops, and Jam Session is the best incoming mixup tool in the game.
There's no real reason not to start Zero with that team. Zero/Magneto/Dante and Zero/Dante/Magneto both make more sense. Putting Magneto on point with no cover other than a Vertical assist isn't really an awesome strategy. Dashing up and calling Jam Session over and over is gonna get you sniped pretty quick. However, Magneto and Dante both have great assists for Zero, who is just as mobile and strong on point as Magnus. But Magneto works way better in the other two slots than Zero. All you really get with Magneto on point is the Shockwave -> Sougenmu DHC, which means you have to wait until you build a bar and then hope your opponent loves doing unsafe stuff in the air or spams assist calls.
And honestly, Nova with Bolts is probably better if all you care about is pure rushdown.
Why is Magneto/Doom/Sentinel not considered better than team FChamp? Arguable best character in the game backed by arguable two best assists.
Magneto/Dormammu/Doom is far superior because it's more flexible, way better at punishing assists and has fewer weaknesses. It's at once the best Magneto and the best Dormammu team. Dormammu + Doom is way, way better than Doom + Sentinel, and Missiles and Drones both take so much time to call that you're not going to be doing a lot of swapping between them.
Magneto can transfer between rushdown and zoning while missiles keeps him from eating a lot of damage at once. Stalking Flare in my opinion is his best DHC and EMD is one of Dormammu's best assists. Even if a team can deal with Magneto, pulling in Dormammu with EMD and Missiles is almost insurmountable for a solid portion of the cast. Plus Magneto makes awesome use of Dormammu's liberation assist.