1. Ashnard to me was the most fearsome antagonist. He cared very little about life and would pretty much watch the whole world burn. What makes this even better, he's human. He's not an evil demon(he acts like one) or an monster, he's a regular person like you and me who just happens to be mentally insane.
2. Medueus in my opinion was the least fearsome. Sure he's an walking giant Earth Dragon that can destroy countries, but we never really see him do anything. For nearly the entire game of FE1 and 11 he's completely inactive until the final chapter.
3. In my opinion make the villain more active, show us the terror and mayhem they can bring. This can make the battle with them even more intense. Also give them human qualities so that they just aren't evil cardboards. It shows that they were regular people who went horrible wrong somewhere along there life.