For me, it often tends to fluctuate. I could say grass-types are my favourite, but that alone wouldn't do justice to all the other types I absolutely love too. I tend to like pokémon design-wise, and not as much for their specific typing, however, grass-types are an absolute joy to have as they're a real breeze to train. A lot of their moves are based around regeneration and taking the HP of the opponent, so I hardly have to go to a poké centre as often as I would with other types during some level grinding sessions, but I also reserve a special place for Electric-types, Psychic-types, Water-types, practically any type I could think of. But yeah, Grass-types probably take the cake on this one.