Opposite of multitasker, a singletasker is one who only takes upon one task at a time, and follows it through to completion. Often used sardonically when someone is bragging about their supreme multitasking skills.
Bobby: "Today I'm only going to focus on finishing my TPS reports and not to answer the phone, email, text, IM, staple, and make breakfast simultaneously."
Jeffy: "You are such the singletasker!"
Bobby: "I can't even respond to you at this time, because then I'd be multitasking."