The Eiffel Tower sells four kinds of ticket, and the only official seller of single tickets is the tower itself, at ticket.toureiffel.paris. An adult pays €14.80 to walk to the second floor, €23.50 to ride the lift there, and €36.70 to ride all the way to the summit, 276 meters up. Online and ticket-office prices are the same; the difference is that online time slots sell out, the summit first.
So the real decision is not which ticket but how you want to go up: on your own with a timed slot, on foot, or with a guide who handles the entry and explains what you are looking at. Below are the official prices, then every guided option we could verify, compared.