So, it is an option that one just admits not knowing the answer which has been stated here a few times worded as "we're small, this is difficult". But I assume those who care to argue against other methods know a lot more than that about rainbow body and how actually it is an unsurpassed attainment.
I'm not a Dzogchen follower, it is simply my interest in Dzogchen teachings that made me bring up this question here. Also, I think, it is good to know what actually is meant by the different schools when they claim to have the best of the best method, for sure I want to know the best method too.
To make your doubt clear:
If the result is buddhahood, then there is no BEST or not; first, buddhahood is beyond good or bad, second, buddha (sangye) means completely enlightened, if there is better or less good, it's not complete.
If we accept two kinds of rainbow bodies, mahamudra's highest stage, dzog rim's highest stage are all buddhahood, then there should have no classes.
Therefore, when someone says some method is the best, he means THE METHOD, which is the path. When he says a certain method is good because it can let you acheive rainbow body, he is not saying rainbow body form enlightenment is suprior than other kind of enlightenment. He is saying the method is suprior because it has an ability that other methods don't have. Simple as that.
If you don't think rainbow body is better, it doesn't matter; but you can't deny it's a special thing which other methods can't do, can you? Now you are saying it's special, but what's the good of it? What kind of answer do you expect from people who are obviously not rainbow bodies? At best we can only tell you things from tantra, from commentaries, from great masters' words, right? If you are not satisfied about that, what else can we offer??
There are many ways to measure if a method is better, but in the end you need to check if a method suit you. That's the most important. You said you want to know which method is the best, I think the one suit you the best is the best.
In my point of view, Dzogchen is objectively supreme because the view is totally direct. we can't actually testify the supriority of paths and results unless we are on them or attained them. Logically we can only measure the views, and Dzogchen got the highest view, so we believe the rest is also high. Same logic as why you trust buddha and his teaching.
But that doesn't mean everyone needs Dzogchen, doesn't even mean Dzogchen suits me.