I eat Subway's rather regularly. I haven't noticed much of a quality downturn in recent years.
However, when Quiznos was a thing, I chose it over Subway's every time. What a shame that Quiznos is no more. I loved that place.
Edit: I don't have a Firehouse Subs within order-able distance, so I can't judge them, but Jersey Mike's is the undisputed sub king, to me.
Only downside is that there isn't one near enough for me to walk to, at home or at work, so I always have to order if I want Jersey Mike's, which makes the meal substantially more expensive than Subway's which is in walking distance no matter where I am.