Sharps is the best in sector, imo.
It has one of the best chains and it's jump attack has a massive knockback.
Arsenal is nice because of its damage.
I also think that in base minitank would be the best because it can both defend as well as attack a base.
Grounder is probably the smartest merc but couldn't handle a rape situation like minitank.
Death hunter is also a pretty good merc because it's not as easy to lure for a push. Though if you do attack with a DH merc you may notice that they easily fall.
Xabaras is not a bad merc, but wouldn't be one of the best. While defending the most efficient way to push them is to trans, I'm not sure if theres another way. If you send this merc on a attack and it runs into someone else it's going to follow the person the whole way until that person hits them or whatever.
Arsenal is up there, but I would only use it if my teammates had it as well. It's pretty hard to push other merc with these things constantly shooting at you but there life span is very short so you have to use them wisely.
In some instances I like to use shuriken. Its spinning jump attack chains quite a lot and it can escape trouble pretty well, but not as good as a minitank.
But eh, everyone has different views.