I have never been up the Fernsehturm. It's certainly a monument, visible from all over the city (and a handy way for me to orient myself). If the lines are long to go up, you can also consider the Kollhoff Tower (red brick skyscraper) at Potsdamer Platz, with it's open air walkway up top, or the Europa Center observation platform, fully enclosed and with a less turbo-charged elevator ride, for those who like the view but not the vertigo.
Since Berlin is flat, all should provide pretty good views, and may have a shorter wait.
For that matter, the view from the Reichstag dome is spectacular too, all the more so for being less elevated than its peers. Though definitely NOT without a wait.