I go to Baker Street every morning, and after some experimentation between trains and the Overground I've settled on the train to London Bridge and then the Jubilee Line. On a perfect day I get the 7.42, and that usually gets me to Baker Street at 8.15 (well, that's when I get off the Tube). On that train I always get a seat - same goes for the next couple of trains too.

(The Jubilee line is less likely, until Westminster, and then generally a seat can be had.)

Coming home I'll generally get a seat too, unless I'm hopping on at the very last minute.

I'm v short so public transport in the whole standing up/squished in mode is quite unpleasant (armpit, anyone?) - and I've found travelling in from Forest Hill to be about as pleasant as it's going to get, really.