I have a tie between two pokemon for my favourite and they are Cyndaquil generation two fire type starter and Mudkip generation 3 water type starter both Cyndaquil and Mudkip helped me though alot during my playthoughs in the Johto and Hoenn regions once Cyndaquil reached both Quilava and Tysplosion are both really fast and high special attack with good physical attack so you can make him a mixed attacker if you want to the best moves I’ve used with him are Eruption and Earthquake with Mudkip it was a rougth start but once it has became a Marshtomp and Swampert he was almost undeafeatable with amazing typing being water/ground, only one weakness being grass and access to moves to help with his weakness such as ice punch and ice beam the best experience I’ve had with Swampert was taking down my friends mega Sceptile with a quick claw Swapert and finished Sceptile without taking any damage. in my playthoughs with Mudkip once he’s reached his final form Swampert was undefeatable being really strong and fast makes it challenging if not impossible to defeat without knowing his weakness