I truly believe that conjuration is the most important skill in Skyrim. This under two conditions. First that you dont have a companion and second that you play in maximum difficulty. In higher levels enemies like draugr deathlords have you with a couple of hits. Unless you have a strong companion with you then you have big problem in defeating boss enemies. Your summoning attracts attention while you heal yourself or retreat. I would vote as second more important Marksman because without it you cannot defeat dragons

I disagree. There is no "most important" skill.

It all depents on your style.