First Preference: Poison
Second Preference: Electric
Poison seems designed to be support, with all the good resistances yet crappy STAB. Being able to absorb toxic spikes, as well as being compatible with many other poison pokémon with good support moves makes this ideal.
Electric offers a single weakness although not as good resistances, but I think that an electric type built defensively can be really useful as a support pokémon.
Second Preference: Electric
Poison seems designed to be support, with all the good resistances yet crappy STAB. Being able to absorb toxic spikes, as well as being compatible with many other poison pokémon with good support moves makes this ideal.
Electric offers a single weakness although not as good resistances, but I think that an electric type built defensively can be really useful as a support pokémon.