Any ideas what the best path for a mythic summoner would be?
Generally, the marshal path, with Dual Path (Archmage), plays toward the strengths of the summoner class: making your (summoned) allies better. Dual Path lets you pick up some spellcasting boosts as well, like Speedy Summons.
A player in one of my groups found "guardian" to work very well. Very tough to take down the summoner, and some of your resilience transfers to the eidolon.