Bah, Grass Pokemon is not weak! Some of them were really good in competitive, like Venusaur, Breloom, Ferrothorn, Roserade, Rotom-M, Tangrowth, Tapu Bulu and plenty more. Their design are pretty awesome too, i mean oh all the new starter, the Grookey line was my favorite.
As for my favorite pokemon, hmm, i don't know, they changed a lot over the years, but few of my top ones were the likes of Gyarados, Machamp, Aggron, Nidoking, Tyranitar, Weavile, Honchkrow and Magmortar.
I personally prefer Pokemon that good in battle, have cool design but not overrated, one of few reasons why i didn't really like Charizard anymore