If you liked the Brave Police J-Decker anime, the Anime-Planet community thinks you'd like:
Fifth grader Seiji was just a normal kid, until he became pulled into a conflict of galactic proportions! A UFO attacked his town one day, and after hearing a voice speak to him, he found himself in the company of a police car-turned-robot named Da Garn. It is one of eight Brave Fighters who have protected Earth since its infancy – all of which became Brave Stones and are scattered throughout the planet. It’s up to Seiji to find and awaken the other Brave Fighters, command Da Garn to battle the alien invaders, and keep his secret safe from his classmates!
Let's face it - if you've seen one Sunrise mecha series you've likely seen them all, as they all are remarkably similar. J-Decker and Da-Garn fit better than usual because they involve a young boy who's somewhat accidentally charged with protecting the world, with a sentient robot by his side.
Both shows are about a kid who teams up with a giant robot who transforms into a police car to fight off assorted villains and monsters. The protagonists in both shows expand their teams over time, so that the kid in each winds up leading a whole team of giant robots.
Deep in the recesses of space, the powers of good and evil are in a constant battle for the cosmos. Earth has remained out of the struggle until now, when the Geisters – evil energy lifeforms – invade the planet and search out host bodies and treasures to steal. Luckily for humanity there’s hope – the energy lifeform Exkaiser of the Space Police descends to the planet and inhabits the family car of young Kouta! With luck, Exkaiser and other transforming robots will help put a stop to the Geisters and save mankind!
While there are plenty of mecha anime out there with different themes, J-Decker and Exkaiser both involve a young person inexplicably becoming entwined in a battle between their giant robot benefactor and the baddies (from space!) With a similar feel and tone, you'd definitely like one if you liked the other.
The evil robot Megatron, the leader of the Predacons, kidnaps Koji’s father, Dr. Onishi, to use the doctor’s knowledge of Earth’s energy sources to take it all for himself. Luckily, a team of good robots disguised as ordinary vehicles are here to help Koji get his father back! Optimus Prime, the leader of the Autobots, has sworn to protect Earth from the evil Megatron. Using the information from Dr. Onishi, Megatron sends his Predacons all over the globe looking for energy, but the Autobots fight to stop the Predacons while not harming any humans. During these battles, Megatron must deal with an incompetent second-in-command named Sky-Byte who is more concerned with impressing his boss and writing poetry than following orders properly.
Both of these shows are about a kid who befriends & aids a group of sentient giant robots. The art and animation are very similar in both series, and the two just "feel" very much alike. If you liked one, you will certainly enjoy the other.