There are essentially three routes you could go with this:
1. The famous, influential person who changed the shape of the world. They might have formed new empires or invented or thought of something incredibly significant. Examples of this include Genghis Khan and Alexander the Great (for the side of leaders), and Albert Einstein and Confucius for revolutionaries in thinking.
2. The broadly known, popular person who changed the culture of the world. They might have changed the way people dressed or talked, or were significant authors, musicians, etc. Examples of this would be people like Geoffrey Chaucer and William Shakespeare, Beethoven, Charles Dickens, John Lennon, etc.
3. The personal, personally known person who you knew well in your life. They might have been a sibling, parent, grandparent, or someone else close to you.
For me, I wouldn't bring back someone I personally knew. They had their death and closure, and I'd find it selfish to bring them back just for my personal happiness. I also wouldn't bring back a conqueror, since that's the last thing the world needs. That would basically narrow it down to great thinkers and great cultural icons, and most great thinkers have done their most important thinking by the time they die; so I'd go with cultural.
I suppose I'd bring back John Lennon. His death was far too soon, and incredibly tragic. Not to mention he produced some of the greatest music the world has ever known, and it would bring happiness to so many people. And I'm sure there are more significant people to bring back, but he's the best example I can think of right now.
