The SSG has the most utility of any gun in doom2, and can be used in a lot of situations. That being said, it's strongest in an open arena, close-range, mid-range, hallway surprises. Most players choose maps that are open, hence making the SSG the best weapon to choose most of the time. Also most players enjoy playing aggressively and close the distance between themselves and opponents quickly, making the SSG also the best weapon to choose most of the time.
Other weapons are good in their own specific instances. Chaingun is a great weapon for long-range, especially when you learn the tapping technique to make it accurate. If you want to annoy your friend and show them that the SSG is not overpowered, play a map where you can get a chaingun and sit at range, slowly tapping their health down while you back-up and maintain long-range distance. My bet is that your friend will be like "dude, stop being a coward and running away, that's so annoying" instead of accepting that the SSG is not as overpowered as originally thought! :)
Rocket launcher is a good controlling weapon in tight spaces. Plasma is also very good at controlling tight spaces and deals a ton of damage if you catch someone. BFG is good for controlling your opponent and taking advantage of their mistakes if they misplace themselves. Shotgun (single barrel) is good at long range as well.
If you want to take my chaingun troll even further... grab a chaingun and BFG. Keep long-range distance, tap your friend down. When you see them getting ready to rush you, switch to BFG and blast them with a cone off a nearby wall.
TLDR: SSG has the most utility and good in most situations but every gun has it's pros and cons, but most maps are built in a way that benefit the SSG, most players play in a way that benefits the SSG. Want to get the most out of each gun? Create situations that are good for that specific weapon.
