Any of these words will get you directed to the necessary place if you ask "Where's the X?" in a restaurant anywhere in the world. However, the meaning is not always the same for all native speakers, particularly for British and American English speakers.
They all have one or more of the following basic meanings.
1. An object: a receptacle which we use to relieve ourselves of bodily waste.
2. A room containing #1 (usually with some handwashing facilities)
3. A room containing a bathtub and usually a washbasin, shower and probably (but not necessarily) #1.
Now let's look at how the words are (or aren't) used by American and British English speakers:
Toilet: Only #1
This is why you should not say "Where is the toilet?" or "I'm going to the toilet" in AmE contexts. It seems quite direct and vulgar. When an AmE speaker hears the word 'toilet', they think of the actual toilet bowl, so it doesn't sound very polite.
Bathroom: #2 and #3
For the reason above, AmE users tend use the word 'bathroom' in most polite situations. If an AmE speaker asks where the 'bathroom' is, they're probably asking to be directed to a room containing a toilet and washbasin. Can be in a public place or private home.
Used in the same way as 'bathroom' (meaning #2) above, but used more for public places.
WC: not used
Loo: not used
Toilet: #1 and #2
This is why it's OK to say "Where's the toilet?' in BrE - it's the room as well as the object. It's a little bit direct, but quite acceptable.
Bathroom: only #3
A BrE speaker would only usually use a 'bathroom' to take a bath or shower.
Restroom: not used
WC: #1 and #2
Rare. Very outdated and never used in conversation. You may occasionally see it written on signs and doors.
Loo: #1 and #2
Typically British and very widely used. It's a friendly, polite and informal alternative to 'toilet'.