Ben's favorite Aliens for each series.

OS - Fourarms (Ben relied on him a lot for some reason. Personally, I think an actual 10yo would've spammed XLR8, Heatblast, Daimondhead and Ghostfreak instead, but I guess Ben is just built different)

AF - Jetray ( Ben seriously overused this guy back in AF. Glad UA and Omniverse toned him down)

UA - My guess is Rath maybe (UA was pretty balanced in not giving certain aliens too much screen time than the others)

OV - Definitely Shocksquatch (Don't let Ben fool you with his constant begging for Humungosaur. Shocksquatch was secretly his favourite, due to how often he appeared in the series)