One of my last memories of 56s - in their last few days - was watching one move a heavy train of HAAs from a standing start at Wrawby Junction towards the Lincoln line...could hear it powering away for at least five minutes. That was the last time I ever saw one (under its own power at least) on the mainline . Did finally get haulage behind a 56 (006) though - even if that was only a quick shunt at 'Railfest' last year .
Very rarely got to see or hear 56's down here in the middle of Wiltshire but you certainly knew when there was one about. The good news is they may be back, Jarvis have bought 4 for infrastucture trains (two for refurb and two for spares / scrap). FM rail also has five, one of which (56125?) is currently being fitted with OTMR and TPWS. What are the chances of a 56 hauled raltour next year?
I'm still waiting for an answer to Nick's request for a definition of 'hellfire' from 6M60...
With regards to 56s, I never have seen one working properly in reality, seen clips of them on DVDs and stuff, but what I have seen there was nothing special. Indeed, the clip of 56097 (IIRC, might have been a different number) on the Nene Valley showed how badly the track wanted replacing IMO!
56s back on the mainline will be interesting, I'll be eager to see how well they work.
So to answer the poll question, as there is no suitable answer for me there, I'll have to say the 60s are the one freight loco class that almost guarentees hellfire performances. Indeed, coming off the incline on the Severn TJ side of the Severn Tunnel can get some interesting results, more so if it's held at the signal near the top of that incline!