Hard to choose just one as I have several favourites, I really like both the Cyber Sleuth games and Digimon Survive too, I also really like the original Digimon World on the PS1 as well, it can be frustrating and repetitive at times but finally completed it for the first time last year, my last Digimon I defeated Machinedramon with was a MegaSeadramon named Derpy (I named each one of my Digimon's incarnations after a different MLP character, had to abbreviate some of their names a bit to fit the 6 character limit though). Probably more nostalgia on this last one but I also have a soft spot for Digimon Digital Card Battle on the PS1 too.