I've been favoring a speed asset and HP flaw. In my opinion, speed is the most important stat in the game. It's crucial for damage, improves avoid chance, and helps prevent being doubled. My first MU was a magic asset, strength flaw, but his speed was on the low end until I fed him a few speedwings, and the decreased strength wasn't great for Ignis. HP is sort of a matter of personal preference - I find it less helpful than Def, Res, or avoid %, for the most part. It's been working out so far.