Why 15 years…shoot for 200 and they can just inject a batch of synthetic stem cells programmed to rebuild/replace your existing organs without invasive surgery.
Mundane; go back to 1982, Cupertino, CA. And meet with Jobs and Woz, show them the current iPhone, and Mac hardware, and tell them “oh, by the way, there’s a little company that will want to meet with you called “Microsoft” run by a guy named “Bill Gates”…
…whatever you do, DON’T talk to them!
Serious; go back to August 2001 and warn the military about OBL and the impending terror attacks on 9/11, try to steer the timeline to a better outcome where 3,000+ people don’t lose their lives and send humanity screaming down a worse timeline.
Selfish; go back to my high school days and tell my younger self to avoid the decisions leading up to my current crappy timeline.
If the above doesn’t work, go back to 6 a.m. on 9/11 and call in a bomb threat to the WTC buildings, warning that they must be evacuated immediately.
The future? Say June 20, 2109. I could pass as some old fart confused by all the modern stuff (I do that now) and still get a look at where the last hundred years got us.
Past: March 1 1754 around the area where Fort DuQuesne would be built – just to watch it all unfold. I may stick around until 1758 for its destruction in December 1758.
Are we allowed to change history? If so, I would be strongly tempted to go back to 1889 and murder the infant Hitler in his crib.
Try to foil John Wilkes Booth, Lee Harvey Oswald, Gavrilo Princip, and Pearl Harbor.
If not:
Golgotha is going to be too crowded with time-travelers trying to get film of the Crucifixion and the Resurrection. I would try to get footage of the Sermon on the Mount. Or perhaps try to get a personal interview with J.C. to see what he really preached.
I would like to film Homer, performing the Iliad and the Odyssey. Shakespeare’s plays, first run. William Gillette performing Sherlock Holmes. Boris Karloff performing Arsenic and Old Lace.
Pericles, Demosthenes, Cicero. The Gettysburg Address.
Get some film of Kennedy’s assassination, from an angle where the grassy knoll is visible.
Assuming we could choose the place as well as the time, Rome about 100 AD, at the height of the empire. What an energetic and amazing place and time it must have been.
So you are FreedomFighter69! The truth at last.
I’d like to explore my city in the late 1920’s.
To all the people who want to visit ancient Rome, Greece, Palestine, etc … don’t forget to learnt the language first. 
I’d like to take in a baseball game where Walter Johnson was pitching. Or maybe one where Ruth is in right and Gehrig is on first.
And I’d be really tempted to go to November 22, 1963 and train a video camera on that window of the book depository.
[Del]I’d go back 6 hrs, or how ever long, after a huge lottery drawing that had no other winners.[/del]
I’d go back and murder cher before she can record “if I can turn back time”
I got nothin’
I’d go back to about 1845 to near Sutter’s Mill in California. I’d fill my pockets with gold and return. It wouldn’t be enough to stop the gold rush or anything, but it would be fun to do. Maybe I’d collect up a bunch of pre-1964 nickels and go back to 1964 and exchange them for uncirculated silver dimes or quarters.
September 28, 551 BCE. Get rid of Confucius. Sorry to those who admire him.
Damn, that’s classic Twilight Zone irony. Great answer.
Make sure you get your cholera vaccine first.
I’d also like to visit London’s Globe theatre to see Hamlet and have a chat with Will.
Just to see what would happen:
Sept 13, 1862: Pick up the cigars D.H. Hill dropped before Antietam
Sept 26, 1983: Swap the analysis papers to show Petrov Stanislav the missile attack was NOT an error
August 28, 1776: Make a lot of noise to alert the British that Washington is sneaking away from Brooklyn Heights
Channeling Homer Simpson:
May 20, 1983: Somehow get into a media-covered premiere of Empire Strikes Back, and tell the TV newscasters outside “Who would have known that Darth Vader is Luke Skywalker’s father?”
I’d go back to Florence, Italy, around 1504-1505. Hopefully I’d run across a bunch of great artists like Leonardo and Michelangelo so I could fawn all over them.
It would also be interesting to visit the Little Bighorn River, Montana, on June 25, 1876 and see exactly how the Custer Massacre played out, but I’m not sure I have the stomach for that.