In rough order of goodness, with real photos from my bakery escapades
Margot Bakery / Holloway Model Bakery (same owner) - the ultimate place to get sourdough sweet baked goods such as sourdough pain au chocolat or babka. Their challah and seeded sourdough loaves are also excellent. Also the best savory (e.g. spinach) stuffed pastries I’ve tried.
Miel - French-style bakery, probably the best of its type in London. I had an excellent quiche there (not my usual choice, but it looked excellent and I was not disappointed). They use really high-quality ingredients, particularly their chocolate and butter choices.
Jolene - make the best pain au chocolat I’ve tried in London, also do a good soft cinnamon bun and various seasonal specialties, such as pumpkin cheesecake.
Sourdough Sophia - great cheese pretzels, sourdough bread, croissants, and Basque cheesecake.
Violet Cakes - best banana bread I’ve tried, as well as great chocolate, halva and tahini brownies.
E5 Bakehouse - really great bread, particularly their rye loaf.
Ottolenghi - best almond croissant in London, plus a variety of unusual cakes. They do a great pumpkin cheesecake around autumn.
WA Cafe - Japanese patisserie, with awesome fresh traditional Japanese cakes and breads. Their seasonal specialties are delicious, especially around autumn when they do pumpkin / sweet potato things.
Bageriet - small Swedish bakery, with delicious sweet Swedish cakes and buns.
Kova Patisserie - another Japanese bakery - order the sea salt lava chiffon cake, mille crepes, or Japanese cheesecake.
Aux Merveilleux de Fred - best warm chocolate chip brioches.
Pophams Bakery - specializes in croissants / buttery laminated pastry - their maple bacon swirl is particularly interesting; they also serve this warm.
Maitre Choux - the place for anything choux pastry related - they produce many kinds of fancy eclairs. I particularly like their chocolate choux bun.