Plainsrunners have really, really good bearcharge. I believe mountaineer explorers prog bearcharge in all terrains and swampdwellers prog dirt kick in all terrains. With explorer you can pursuit in all terrain. Explorer herbs give the bonus in all terrains as well.
Basically, the only 3 legitimate options for explorer are caverndweller, marshdweller, or plainsrunner. The former gets all kind of extra attacks and dodges, the latter two get pursuit. Marshdwellers lose bearcharge and plainsrunners lose snare. There are a lot of ways to beat snare and a lot of ways to beat bearcharge, but it just depends on your planned opponents. If I was Nexus, I take plainsrunner in a heartbeat. Caverndwellers suck against riposters or concealers. Snare crushes conjurers, and bearcharge does not. Bearcharge is a lot more likely to manage to beat an invoker than snare is. Bearcharge is a lot better against a paladin than a snare. And so on. Decide your opponents first before picking.