For me:
Proxy: A fast running engineer with those lovely mines. When it comes to rushing the objective, or just hiding behind the EV to repair it, this is the preferred merc for me. (Game mode experience)
Nader: Granades clear rooms and mob attacks. Sometimes they can even take out distant snipers, or at least suppress their fire for a moment. And the suicide is not that bad either, although people are starting to learn how to avoid it. And although it is not the ideal, the granades can also stop the EV. (Plain old fashioned kills)
Skyhammer: Ammo packs usually net me 280 points of support when I spawn to an area, thus making the support missions one of the easiest to complete. Airstrike can at times wipe out the entire enemy team along with the EV, and the riffles do reasonably well with distance. (Support experience)
These three mercs are what I prefer to use above all. At times I feel almost insidious when I place the mines. Not to mention how I am starting to get a hang of the granade launcher flight arcs bounce angles. (Though my aim is still ways off from what I had during the old W:ET days, I loved those engineers with their granade riffles.) And yeah, airstrikes. (Bit sad that you need to be alive long enough for the strike to go off, no suicide runs for me with skyhammer.)
Additionally the above squad enables me to fulfill almost any role in the team. Well, the composition is short of a medic… But, medic mercs seem to be popular with other players. And why wouldn’t they be when in skilled hands those are almost impossible to kill?
So, what is your favourite squad? Why have you gone for it? What perks it has for you?
But joking aside, if you learn to do direct hits with grenades and use level knowledge to shoot grenades blind of walls/etc, she becomes a killing machine. Martyrdom is just icing on the cake (shoutout to all Phantoms!). She’s one of the mercs that’s easy to learn but hard to master (I’m not claiming that I mastered playing her!).