When comparing 100% (Complete Forme) Zygarde and Kommo-o, Zygarde is the best by far. All you need to do to get it is to collect all Zygarde cells and cores. Then, select which Forme it should start out on (Doing this and letting its health drop from full to half is the way to get Complete Forme Zygarde). Let's take a look at which is better stats-wise.
Complete Forme Zygarde's BST (Base Stat Total) is 708 whereas Kommo-o's BST is 600.
Complete Forme Zygarde's HP is better than Kommo-o's (216 vs 75).
Kommo-o's Attack is better than Complete Forme Zygarde's (110 vs 100).
Kommo-o's Defense is better than Complete Forme Zygarde's (125 vs 121).
Kommo-o's Special Attack is better than Complete Forme Zygarde's (100 vs 91).
Kommo-o's Special Defense is better than Complete Forme Zygarde's (105 vs 95).
Their Speeds of 85 are the same.
Based on this, Kommo-o's stats are better (excluding HP).
When weaknesses are concerned, Zygarde and its Formes (Dragon/Ground) are 4x weak to Ice and 2x weak to Dragon and Fairy.
Kommo-o (Fighting/Dragon) is 4x weak to Fairy and 2x weak to Ice, Flying, Psychic, and Dragon.
Even though Kommo-o has 5 weaknesses, it's stats (again, excluding HP) are better than Complete Forme Zygarde's.
