I make muffins most Sundays, as I have the oven on and it's the only time I really 'bake'. It's my 'happy place'! 
My go-to is banana, as I usually have bananas in the freezer - only DS2 eats them but only at 'peak ripeness' for him, which means that i put over-ripe in the freezer; they are ideal for muffins. Mis it up with nuts/choc chips or both.
BUT - I have no bananas! I made chocolate with choc chip last weekend. I've previously done orange, carrot/cinnamon, apple, pineapple...
I have cherries in the freezer, and also a bag of mixed berries. I have a tin of pineapple. I have apples, and carrots. I have cooking chocolate bars and coffee - no cocoa powder tho. I have all the spices and stuff, inc ginger in the freezer.
What's your favourite? I can always pop to waitrose later if something piques me! Gotta be sweet tho, Bran, herb or cheese is not going to go well with custard 