The Heart of the Matter is my favourite of his books. The Comedians was also one of the ones I enjoyed the most but I read it when I was very young, so much so that I cannot remember any details...which of these should i read next?
Although one thing suddenly came to me, I remember my mum taking me out of school to go to the dentist (must have broken a tooth) and the dentist saw me carrying The Comedians and we had a conversation about how GG was his favourite author.
Not exactly an earth shattering story but it's interesting that it popped into my head from nowhere like that, haven't thought of that guy for years but that incident must have been stored pointlessly in my memory, lurking, ready to pounce if I thought about that book.
I think The Human Factor is good too, in fact my opinion is precisely the same as that of @jenks in that the only inferior GG I have read was Travels With My Aunt... it was fine, but wasn't what I wanted from him. I don't think I read any of his other light-hearted ones.