Green Mars, Blue Mars
I think Green Mars might be the best of the three.
One big reason is by the time you read Green, you are already seven or eight hundred pages in, so if one of the colonists decides to take a rover out in the desert for some geological surveying and deep introspective thinking for a couple weeks (and twenty uninterrupted pages), as the reader you are by now used to that kind of thing happening from time to time, and it's no longer so jarring to be separated from the other parallel plot threads for so long.
The other reason is simply that by the time you slog through to the end of Blue, you've been reading this story for so fucking incredibly loooong...
But all three of these books are great.