My youth is clearly showing here, but I have a lot of nostalgia for Voyagers. When I was little, for whatever reason I thought they were the coolest trains around. I guess it was a combination of them being able to tilt, the engine sounds (I love the sound of a powerful DMU, it's the same with the Adelantes), and the pleasant interior (when there's no overcrowding, I still maintain that Voyagers are really nice when they're not busy). Whenever my family went on holiday up North I would always get excited at the prospect of a 4 hour journey on a Voyager, something I doubt many on here can relate to!
A more conventional answer would be the HST. Just hearing an MTU engine makes me feel nostalgic. I'd say the same for the Valentas but I'm barely old enough to remember them. Most had been converted to MTUs by the time I was 8 and I only have vague memories of seeing and hearing the last few Grand Central HSTs with Valentas around York. One other HST sound I'm nostalgic for that I already miss is the sound of the doors slamming on the Mk3 coach. It brings back memories of travelling with my parents for holidays in London.