Spellcaster.

What? They tend to be heavily protected and mostly avoid physical labor. And it would make happy to cast a spell.

Seriously though (ha!), what role I would fill would be very dependent on the composition of the group, the tactical situation, and any identified strategic knowledge. Group composition may clearly determine if I am the strongest or smartest or whatever and force me into a role. That is, a group dominated by the very old and/or sick or with small children/infants would have available roles that should be very different than a group comprised of young adults with say, military training. Open roles would also be defined by the tactical situation. Such that a group exposed in a mobile situation (running through woods filled with fast zombies) has different roles (ooo, many more openings for ‘fodder’) than one holed up in a defensible position (a mall, ha!). Finally any available strategic knowledge would dictate available roles since that knowledge would direct the longer-term group/individual goals over immediate survival. Like are these zombies going to rot out in a few days or otherwise follow real-world physical laws? Is there someplace to go that is better than where the group is? Etc.

So really it is hard for me to say in advance. True, if one has a very specific skill(s) like medical or military, you can probably guess that is what you would doing in at some level. But even then the doctor/nurse might the strongest/fittest person in a given group and thus is the main door-kicking fighter or the military person might be the only one with medical skills in a group full of hurt/sick people. For me, I can’t know in advance what roles would be open without more context. But if I get to choose then:

Spellcaster. Definitely Spellcaster.