Inisghts into Holygrail vs Foundation vs Multiflex (vs Mollio)

I am looking for opinions on working with the above themes as the foundation for my own theme customization efforts. I want to learn one and use my knowledge of it to build 3-5 additional sites that will all look very different once I am done.

I have kind of ruled out Mollio, because of the need to manage two stylesheets and keep them in synch (one for IE6 and below, and one for all others.)

That leaves Foundation, Holygrail, Multiflex as the top contenders.

Needs:
Working with 4.7 and hope to seamlessly go to 5.0
Need something that lets me easily go from 2 to 3 columns
Prefer liquid site - or at least a liquid center content.
I intend to heavily leverage other drupal modules, especially forms & polls, events, etc.
Want it to support as many Drupal options and hooks as possible (breadcrumbs, mission, slogan, etc.)
Want one that is going to be continuously maintained, updated, improved by Drupal users who value it.

If anyone has an opinion as to which of these might suit me best for these needs, I would love to hear from you. - Or if there are any key differentiators between these themes and how they are implemented, I do not see what they are at this point, and would love to have your insights.

I have reviewed them all, and they all seem nice to me. I think Multiflex has the code comments, which is a plus.
Other than that, I am stumped.