It all depends where you are. 'Toilet' and 'lavatory' refers the the object itself, but in polite company - especially in the USA - people prefer not to use these words.
WC originally meant 'water closet' (toilet) but nobody ever says this. The initials (WC) are used mainly in Europe.
Bathroom and restroom are the American terms. If you are in the USA, you would always ask where the 'bathroom' or 'restroom' is. 'Toilet' would be understood, but might be seen as a little indelicate.
In British English, a 'bathroom' is where you take a bath or a shower, and a 'restroom' is where you rest. We are less uncomfortable about the word 'toilet' than Americans are, but if you want to be polite in a public place, you could ask for the 'Ladies' or the 'Gents' (short for Ladies' Toilets or Gentlemens' Toilets). Otherwise, the all-purpose British word to use is 'loo'. British people use the word 'loo' in most situations - with family, friends and colleagues.