There's nothing like a good pub, a haven of chat and humour, a shelter from the typical English weather, a place for beer and relaxation. A good pub doesn't try too hard, it isn't all bells and whistles - it simply 'is'.
Marquis of Granby is such a place. You can feel the character oozing through the walls as you walk in. It is a proper local pub, with an interesting mix of New Cross-ites and students from the nearby University. It is a good example of an authentic Irish pub in London too, no fake pub paraphernalia here.
Here are some reasons to recommend it - it isn't fake, so therefore it's not that trendy either (a good thing), it is a proper boozer, so for those people who want to see what an actual working London pub is like, it's a great example. It is very decently-priced too (another good thing).
It may not be to everyone's taste - it is a local rather than a place to be seen, but the bar staff are welcoming, and this kind of place needs preserving, in an area where gentrification is stretching its claws.
It is the type of place people say is full of 'characters' - by this I think they mean normal people, who watch typical things on the large screens (football, rugby) and typical Irish things too (horse racing, curling). It hasn't changed very much in all the time I have known it, but that's probably the greatest advantage of the place - you know what you are going to get.
If it is bells and whistles you want - head to Central London, or the East End for that matter. If you want a quiet pint, reasonably priced, and a chance to feel normal - my advice, treat it like you're entering someone's house - then you will find it difficult to find better in London.