My favorite types have to be Dragon and Water.
Dragons are badasses hands down. Rayquaza (essentially the strongest mon in the game) was needed to stop Groudon and Kyogre. Palkia, Dialga, and Giratina are rulers of their own dimensions. Shit, don't forget Kyurem, Zekrom and Reshiram were originally ONE pokemon, but that savage was lost in time for the better it seems.
Water is probably the second best type. But how could not love water? If you enjoy the rain, swimming, drinking water or any type of aquatic animal, you should love water types. Also, the contrast from Dragon's exclusiveness is the abundance of Water types. We got very good walls and wallbreakers and their movesets are things to truly worry about. Also I believe all the water starters are the best starters so I'm truly biased.