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Rattata/Raticate is good at the beginning, a level 20 Raticate hits REALLY hard with Hyper Fang compared to most other Pokémon.. It quickly becomes obsolete tough, and you're better off swapping it for something else at level 35+
Sentret/Furret is pretty useless, but it's qt.
Zigzagoon/Linoone has a sleek design, good speed and hits somewhat hard. It's pick-up that's the best thing about him tough, it's just a great ability.
Bidoof/Bibarel is an awesome Pokémon, that part-water really sets him apart from all the others. A Great Pokémon, a great HM-slave and can really pack a punch if you use a curse-set.. My personal favourite
Patrat/Watchog is equally useless as Sentret/Furret, but they are just ugly.
I can't form an opinion on Bunnelsby/Diggersby before I use him.