Gunpowder. I don't know the exact formula, but I would bet money that I could work out a close-enough approximation (good enough to kill and destroy).
In order to be able to make gunpowder, I would have to trade my phone to a local warlord. I don't normally carry anything else I could trade effectively, and there's a good chance I could die from any number of things, including the warlord deciding "fuck you, I'll just kill you and take it, if you knew how to create weapons of destruction you would have some with you". So I would probably die before inventing anything.
But, assuming I don't die, and I manage to keep the secret of gunpowder to myself long enough to start work on other projects, I can then move on to some other basic tech. More refined gunpowder weaponry is a job we can give to the smiths as far as the actual work goes, I'll just be sketching out basic ideas to see if they can make them (the first gunpowder weapons I'd make, as a practice thing, would be wooden hand grenades. Metal ones would be obvious, then I could suggest guns and possibly rockets).
Next invention will probably be the bayonet. It seems so simple that it's shocking how long it took for them to come along. My first effort would be the ring bayonet, I might go for modern-style socket bayonets later though.
After that? The flintlock. An army with flintlock muskets and fixed bayonets will wipe out a mixed arquebus/pike force even more reliably than an army with arquebuses will defeat pre-gunpowder armies.
Only then - by which point it's probably been like ten years, and my liege lord will have probably taken over the country - will I start on steam. I will probably take another ten years to perfect it enough that it makes revolutionary changes, and I might not even succeed.
During that work, which would basically be my magnum opus, I would dabble a bit in things like the seed drill, and a few other agricultural/industrial revolution era inventions. If things are going really well, or a really good opportunity presents itself, I'll invent the semaphore.
With all those things done, in approximately 25 years and assuming I survive that long, I will then basically retire - still making the occasional tech advance, but mostly relaxing with that as a hobby - and chill in the fourth-largest mansion in the country with my twenty-or-so concubines.
EDIT: Oh, also, plenty of hygiene advances, especially the flushing toilet, but that would be more a hobby thing than a work thing - whichever lord patronises me will probably have me killed if I stop giving results too early on.
