Depends on what weapon and magazine(s) you are carrying, and how much other kit you need with you.
Using a hi-cap magazine fitted to your gun and just some spare BBs in a pocket you can just run around (or walk quickly in my case) very light. Or a belt rig (a pistol and mags if wanted) with mag pouch, admin and dump pouch. Keeps you much cooler compared to a vest or plate carrier, less dehydrating.
If you have lo-cap mags or use lots of mags then obviously you want something to carry them securely and with access. I do use a vest or chest rig, as appropriate to load and whether playing in the dark or daylight, although either work the other way around but to be honest the lighter load the better.
As most others have said, you want to feel any hits and be comfortable and manoeuvrable.