Don't know if anyone remembers it but Bristol City away in the F.A Cup (maybe 2006/2007ish?). Finished 3-3 and we equalised with barely 5 minutes to go, may have been Stern John. I have still never heard Cov away fans louder than they were that day. The low roof and old school stand probably helped a lot mind. Was too young to travel whilst we were in the top flight and wasn't even born for our cup run, but for some reason that third goal against Bristol City and the atmosphere in general always sticks out for me.
Aside from that I'd say Dele Adebolas against Coventry, there is a great video on Youtube where you just hear the Cov support roar each time Dele beats a man, then when he slots it home its just a high-pitched scream. Great day