Who is better in your opinion? Who pays the most?
Unity Ads is an ad network, first of all, they launched mediation quite recently. The huge advantage is the ease of integration if your game is Unity-based. If you have around 20-40k video impression daily using unity ads as an ad network would be a good solution for you, however, if there are more, some mediation service is better.
Appodeal is a mediation, they don't have an ad network of their own which makes it unbiased towards any ad networks, easy to integrate, have a decent amount of ad networks and you also have access to exchanges, like mopub through them. It's easy to integrate through a single sdk - I use it in my games, dashboard is nice, basically, you have all stats in one place, immediate payouts is something that helps a lot too.
I've tried Admob mediation some time ago - you have to set everything on your own, too many settings are needed and adaptors, comparing to Appodeal I'd say that Appodeal works out of the box and you don't have to know what networks to connect, it analyses your users and connects what is relevant for you. Admob has less networks. Their network is one of the best for banners and interstitials though, but you can connect it through any other mediation.