Since few Pokemon are completely worthless, I'll assume you're fighting someone with Pokemon in the top 100 - 150. This gives you a spread of utterly ineffective choices.
Luvdisc - terrible stats other than speed, it's ability doesn't help it, and it has a tiny, worthless move pool.
Unown - One move, than doesn't even be useful. Also bad stats.
Shedinja - While Wonder Guard protects in from direct damage, any form of indirect damage will instantly KO it. However, it does have 12 immunities, so you could luck out and win. Of course that means no Sandstorm, Hail, Spikes, Stealth Rock, Toxic Spikes, Leech Seed, Toxic, or any moves of the five types that can hurt it from the opponent. Not likely in a six on six.
Regice - Really fast, but will low HP and Defense. And four weaknesses; Fire, Rock, Fighting and Steel, which are all useful types.
Ditto (without Imposter) - Terrible stats and a move pool of 1. You have to waste a turn to do anything, meaning Ditto's bad HP, Defense and Special Defense easily lead to it's defeat. However, make sure you use a non-Dream World Ditto, since Imposter cause instant transformation, meaning you could end up with something useful.
Egg - Egg cannot be used in Battle. Ergo, it is the most useless thing to use. Also type less, so you can pair it with any other Pokemon.
Minun/Plulse - Both have pretty subpar stats. Can be used for Baton Pass, to help Regice suddenly be a problem. Maybe. I didn't check which stat boosting moves they have. Use one of these if you can't use Egg.
