For me there are no real useless Pokemon if you have some skill a little patience and a dash of luck, any Pokemon can be useful, there are however Pokemon that i consider bad. I have decided to list the top 5 bad Pokemon below:
Snivy evolutionary line, now i am a big fan of the theory that starter Pokemon have to be in your team at least until you become champion of the region and until generation 5 i have found that every starter Pokemon even Meganium (I hate Meganium) have been able to stand on there own and generally fight Pokemon 2-3 levels higher than them and usually win. Now I have started the game with Snivy and found it to be the worst starter Pokemon in Pokemon history. It is just bad especially early in game.
Purrloin evolutionary line, what a useless Pokemon, I have used this Pokemon 3 times and It is just bad it gets beaten by almost everything it is just no good, I do not recommend it to anyone.
Hoppip evolutionary line, Now this is a Pokemon i have given numerous chances to cause im not going to lie i kinda for some unknown reason find it cute, however this evolutionary line is just bad even at its finally stage it has mediocre stats which I could tolerate but throw in a a mediocre move pool and that to me equals bad Pokemon.
Miltank, I find this Pokemon not only bad but annoying, i think there is something in the games code that makes Whitney's Miltank really strong and hard to beat but any Miltank in your possession utterly useless i have used this Pokemon a few times in the past and i follow the same basic idea of Whitney, Attract, Rollout, Body Slam and Toxic. and despite having good stats it just does not hold up against other Pokemon.
Kricketot evolutionary line,enough said really just a bad Pokemon in general