It's surprising how difficult it is to get that last loco or unit. There are several classes that I required an odd one of at withdrawal - or, in some cases, still do (e.g. 357 039), for haulage. I've cleared lots of DMU and EMU sub-classes but I'm not counting sub-classes.
Locos: classes 43, 50, 87, 90 and 91
DMUs: classes 151, original 159s (now 159/0), 165, 166, 168
EMUs: classes 313, 315, 317, 319, 360, 365, 442, 455, 456, 458, 460, 465, 466
IE: class 071
There are also some that I'm pretty sure that I cleared but don't have stock books back that far, to confirm (e.g. the 506s).