Most useless fruit: the Mera Mera No MI cause it couldn't save Ace and his broke a$$ from being gutted by Akainu! Oh...too soon? Jokes
The most useless fruit would have to be Alvida's fruit mainly due to the fact that it's an inferior version of the soap fruit that Kalifa from CP9 possesses. Close second would be the Kilo Kilo no Mi, mainly due to the way it is used. If you're are going to fall from a fatal height with the force of 10,00kg, you better hope to god you have some form of super human durability/invulnerability. Sadly this is not the case with this fruit, as demonstrated by Miss Dumbsh*t...I mean Valentine.
The Kilo Kilo fruit would actually be a VERY strong fruit if it was with a user that can use it like it should be used. First of all, you have make any part of your body any weight, meaning you can train your body to handle that kind of weight hence becoming naturally stronger. When you're fruit is not activated, your body would be so light that you can just jump enormous heights and not worry about floating like when it is activated. When you attack, like someone else said earlier in the thread, you can make that body part VERY light so you can accelerate it faster and right before it hits your opponent you make it the heaviest possible weight and SMASH your opponent into dust.