Another vote for cars, trains, tractors - anything with wheels. The most popular thing by far with my 20mo. Ignore all the packaging nonsense that says 3+ years - proper ones far more popular than more babyish ones here.
Ds2's absolute favourite toy is a combine harvester from ELC (was about £8 iirc).
DS1 will be 20months at Christmas and I'm thinking of getting him one of those magnetic etchasketch things. I would have bought him a garage, for his trucks and cars, but he's already got one second hand.