I pick my MCs based on silly or entertaining they are. If the effects they have are good enough for competitive play, extra brownie points.
A good MC imo is one that can generate card advantage or offer unique utility such as board control, unflipping, antidestruction, and etc. After that, its a matter of the MC having a reliable 60atk if 2 range or 70atk at 1 range. The turn you promote into the main MC and the turn after are the two most critical parts of the game that I look forward to. Having an MC or plays that dont make an impact during these two turns hurts a lot.
Choosing a MC or strategy first doesnt really matter as long as you are happy with the deck and how it works in the end.
Shannan doesn't really do anything until the final turn where you go in the kill. Before then , the opponent can easily play around it by keeping their life high or retreating to the backline.
