It's definitely a subjective thing.
I came to this website a year and a half back thinking about the same question, and this site helped to expose me to different systems. Then, a few months back I finally was in a position to do my own job search, and had certain criteria that each agency (and city) had to meet.
-Wanted a municipal third service agency- No fire based EMS and didn't want to continue with private EMS
-Had to be "progressive" in their protocols. The agency I'm starting with next month gives Zosyn and Rocephrine prehospitally, has RSI capablities, gives Heparin for ACS and has RSI capabilities after a year working there. No where near me currently can do anything like that.
-I wanted an agency that valued higher education, I worked hard to get my bachelors degrees, so I was hoping for a place that would appreciate (and compensate) the higher education
-Preference on tiered ALS/BLS system. I think we should be able to send BLS trucks to non-high priority calls, instead of all ALS systems sending medic trucks all over
-Was hoping for an agency that had a better career ladder then just FTO or field supervisor- I think that's important to retention in EMS
-As a medic, I really didn't want to be placed as an EMT again to start like Boston EMS, Austin Travis or Medic One out in Seattle.
-Not mandatory, but I really enjoy working HAZMAT. I found several agencies that had EMS work on the HAZMAT teams, and weren't fire based systems. I liked that.
I also had some criteria when looking at cities the agencies worked in.
-Wanted a medium to larger sized city
-Had to be warm (ish). I was trying to get out of the cold, miserable snowy northeast.
-Had to be within an hour/hour and a half of a major airport
-I preferably wanted to be able to live downtown/close to downtown
There's definitely more criteria, but those were the major ones.
But that's what worked for me! I realize that there's a lot of differing opinions on this website about what works best for others, what I wanted isn't necessarily what everyone else wants. It would do you well if you're doing a job search to make a list like that. Find out what you want, what you like and don't like. Then go from there!