Some of my favorites:

• Gurren Lagann - Some plucky youth rebel against the systems that have forced humanity underground. Very over-the-top
• Brave Bang Bravern - Extremely comedic and extremely gay, this is the story of humanity fighting against an alien threat with the help of a Super Robot who appeared out of nowhere seemingly from an entirely different genre of anime
• Eureka Seven - A boy joins a band of rebels to fight the world government in their hover-surfboard-riding mechs
• Vision of Escaflowne - Of the two prominent 90s mech shoujo isekai, this one is by far my favorite. Who knew putting a cape on a mech would make it look so gallant?