I rarely use trainers myself. but if i had to chose. i would give props to 0x90 for the Mass Effect Legendary Edition Trainer. And caliber for the Halo Master Chief Collection Trainer. and Devil for The Riftbreaker Trainer.
I should have clarified... "favorite" isn't restricted based on your usage. It could be for some technical reason, some eureka moment in development. Perhaps a surprise in a game's attempt to avoid cheating that you were able to overcome. Or namespaces that made you laugh while debugging (silly var names). Or fan response. Or even using a trainer in a game that made the experience of playing that game just so much better *chefs kiss*.
I leave the question open ended for you to decide what and why something is your favorite.