I guess most people will model a period, but like many others I know my own preferences are a bit eclectic, like Doug.
I like unusual items, such as the Lima military train and the old Triang Wild West engine and some (not all) of the Battle Space range.
I have real experience of diesels and electrics, so have some of the locos I drove and would probably have a sample of each if they were all available. But, I prefer the hypnotic effect of watching valvegear and motion on a moving steam loco.
I have an interest in the historical part of railways, so I like the landmark locos such as City of Truro, Stirling Single, 10000-10001 etc. Prototypes are fascinating, so Princess Anne, DP2, Fell, etc are also favourites.
So, even though I don't focus an any particular time period or area, I still get fun out of model railways. As its supposed to be a hobby for enjoyment, you play with what you like and I'll play with what I like !!
PS. If anyone made Duke of Gloucester I'd be first in the queue - 90 mph shovelling coal is an experience I'll never forget !