If we're talking Obito's, I'd give it to Kamui. If it's Kakashi's eye, it's Amaterasu. Both Kakashi, Sasuke, and Itachi in the past have shown that the gathering of chakra takes some time, even if it's minimal amount of time, is necessary to use their MS. Kakashi in more cases, it takes him what seem like a longer amount of time to gather, but in other cases his seem instant (Susano'o, Deva Path nail, etc). More often though, Amaterasu doesn't have a long build up and always travels quickly, while Kakashi was taking a while to Kamui the Mazou head and couldn't gather chakra in time for a Juubi TBB.
Kakashi is more inconsistent with his so I give it to Amaterasu > Kamui but only in Kakashi's case. Obito's Kamui seems to work faster and take affect on others and himself faster.