Most people say Treecko and I would too. There is just one problem. Treecko can be obtained in the post game at joyous tower. I recommend Elekid. It can learn moves like thunder punch, thunderbolt, along with brick break and psychic via TM. These moves can be quite helpful during some dungeons and boss battles. You can recruit Elekid really early. It can be recruited after you clear the first thunderwave cave mission. Since you get it's friend area for free, you can go back to that dungeon right away. Since it's recruitment rate is 10.3, you shouldn't have that much trouble getting it. However, you will have to search for TMs, since they can't all be obtained right away.
Some other options are shroomish and oddish. They can provide coverage for your team. Shroomish's friend area is mushroom forest, which you can buy for 500 poké. Oddish's friend area is Jungle, which can be bought for 800 poké. Both Pokemon can be recruited at sinister woods, the fourth dungeon in the game.
Hope this helped.
