What is the best way to fix this garage floor?
Ahh, the ancient quest for flatness.

You could put down a mortar bed, but it will degrade over time if it is unsurfaced.

In general, putting good concrete over bad concrete doesn't work. Trying to flatten just one area of a garage will not be easy. One possible procedure would be to get a jack hammer and remove about 1 to 2 inches of concrete everywhere you want to be flat. Then get down on your hand and knees with a level and chisel it all out so it is pretty smooth and level. Then pour new concrete into the draft, or better yet get an experienced paver to do it. Making really smooth flat concrete surfaces takes a lot of skill.

Also, you could do all that, and whatever caused the crack in the first place will crack your new concrete.