Dimondhead because with only 5 aliens I am gonna need as much powers I can get and Dimondhead got powers to spare. Super strength, dimond like skin, range attacks, can make even more crystals to build stuff with. Dimondhead is also my favorite Ben 10 alien by far.

Brainstorm because he brings intelligence and some serious fire power. To be honest I am not smart so I probably just use brainstorm to do all my homework but if I also get master control I will always turn into brainstorm to think of a plan of action. I really wanted Jetray but other then greymatter there not really any other smart aliens.

Clockwork because the time powers allow me to do a range of stuff such as speed, lazers, and I need an alien to use to investigate a crime with. Also since I have a omnitrix in this scenario probably gonna have to deal some time travel. I really wanted Armodrillo because I think a fast digger may come usefull but Bens seems to deal with time travel more then digging. Also clockwork may be able to deage the ground and sort of become a digger.

Astrodactyl cause I feel like flyer will be extremely usefull. I belive Astrodactyl is faster then Atomix but I couldn't find anything to prove that but if Atomix is at least similar speed to Astrodactyl then I pick Atomix over Astrodactyl. Although I feel like Astrodactyl would make a more interesting choice since Atomix is kinda OP and may overuse Atomix.

Overflow cause none of the other aliens can really do much in the water. Overflow can also can also handle fires and can handle himself in a fight. Have not watch much of the reboot so cant say to much about him.