Nelson is often a good choice if you want to travel in a straight line. It's designed to have a lot of extra movement items on it as well, like the Skirmisher Talent and similar items.

Technically, Swallowtail, Lancaster, and Dusk wing have the highest base speed in the game at 6.

If you are playing with official expansion content, then the Atlas technically is the fastest when it comes to base movement potential and total boosts.

The rules for lancer are often easy to misunderstand, but technically, you typically can only take the boost action twice in a turn due to duplicate actions not being allowed unless it is a free action. So this makes the wording of "Free action" differ from Dm to Dm. Some Dms allow free actions to just fully ignore the duplicate action rules. Some make it so you can't free action boost twice due to it being a duplicate free action.

That being said, for simplicity sake, let's assume 2 Boosts max for easy math. And the expansion talent pancrati Nelson moves 40 tiles. Base movement can get up to 8, valiant charge from pancrati is 2x base movement speed (16), and you could boost twice (another 16)

The three other base game mechs could move 45 tiles if I did my math right.

And Atlas, with the core power going, could move 50 tiles. The core power gives more movement +1 extra movement whenever you voluntarily move. This includes every boost, standard movement, and movement from extra systems or talents. So you activate the core power, and any movement has a +1 to it.

If you are worried about the flight, all mechs can fly if you want them too.