Southern Region slamdoor electrics. When I was commuting to school (Andover-Salisbury) they were still being run on the electrified lines out of Waterloo, but I wasn't a rail enthusiast at the time, so I never ventured onto one. I became an enthusiast about a year after the Lymington units were finally replaced
. I have been on the Hastings unit a couple of times (what a fantastic train, I must say!), and I've done Southern EMUs hauled on heritage railways, and I've done 4TC. But not one under its own power. Hopefully that VEP will eventually be mainline certified...
To everyone bemoaning not having been on Thumpers, it's not too late to go on one of the Hastings Diesels railtours! They really are fun.
442s pre-GatEx refurb (first class compartments!)
I've definitely been on A Stock from time to time, but not since becoming an enthusiast so I regret that.
A Pendo drag.
I don't think there's much else that I realistically *could have* been on that I haven't. I've done the Devon class 57 day trains, and the 121s, and the Wareham to Swanage 33/37 runs last year.