Rather based on just stars .. Maybe MVP based on the best attack taking into consideration level difference
Remember to Pillage before you Burn
I believe its better if mvp is given to a player that give the most stars in war, but of course since you compete against another player in your team, i guess destryoing persentage could take a factor in it, and also rather giving gem wich can cause player to take advantage over it negatively, i think it is better if mvp gets 10% bonus in war win bonus, this will encourage most player to do their best and willing to try to take down tougher opponents to get better loot bonus.