Socks
Most people wouldn't suspect it, but Mycroft has cold feet. Not metaphorical cold feet; he's never struggled with making a decision or committing to a promise. His feet are literally always cold. Sometimes when Sherlock was small, he'd climb into bed with Mycroft, and if Mycroft wanted him out, all he had to do was put his cold feet on Sherlock.
Mycroft is always properly dressed from head to toe, and would never dream of wearing heavy wool socks with his three-piece suits. But on a winter night, when he's home alone, he likes nothing more than changing out of his lightweight trouser socks and into heavy wool socks. Mycroft believes that there are few things in life better than stretching out by the hearth wearing wool socks, with one's feet warmed by the blaze.
Mycroft Holmes loves his socks.