My personaly choice would be either IV or V. I though both of these adventures were amazing, but could be increased greatly with some engine updating.

I & II I don't think were well enough developed to translate well into the modern generation.
IV & V, like I said had amazing plots but did suffer through some technilogical issues. (They still looked great, but there is room for improvement)
VI & VII, although technologicaly I think they could use updating I think that might ruin quite a bit of the style of the games, and ultimately make them less enjoyable.
VIII just sucked
IX does quite well without a graphical update, although a resolution update would be quite nice. I don't think that is something I would want to pay for though.

I'm against any re-makes that actualy chage the games themselves. If you are going to do that you should just make a completely new game instead.