What is London's biggest or most famous underground metro station? I've been googling these to no avail. Thanks in advance!
Biggest or most famous aren't really quantifyable... I'm not sure, possibly one of the underground stations attatched to the major rail stations? Paddington, Victoria, Waterloo, Kings Cross St Pancras, Euston/Euston Square, etc. Is there anything in particular you're looking for in a tube station? I take the tube every weekend across London, when visiting my girlfriend.
I went to London a few years ago and I have ridden the Underground (only way to get around) You could use Piccadilly Circus. There is an Underground station right underneath the Circus with four entry/exit points for each of the branching directions (Regent St being one of them, which leads to Oxford St, the shopping street of London). This is the station I always headed to every morning.
Same goes for Oxford Circus, really, which is the main access for Oxford Street- the main centre of shopping in London. Picadilly, is a fairly big one, which is the nearest stop for Haymarket, and others. Also, Leicester Square would probably be another famous one.
Just be aware, there is no substitute for personal experience. I could google Park Street Station in Boston's subway system, but it wouldn't give me much of a clue what the station is really like - the smells, the way the crowds move, the musicians playing for spare change, and the screech of the rails when trains are approaching. It's a very different atmosphere than, say, the subways under New York City.