If you're in your late 20's / early 30's ideally Kreuzberg is where you want to set yourself up. While previously home to a large mix of leftist punks, anarchists, LGBTQ+, creative artists and Turkish immigrants, it's rapidly gentrified.
The central district of Berlin and the bougiest. Long gentrified, the clubs picked up and left in the early 2000s and now most bars are either a scene, cocktails, or hidden. It's great for shopping, sights, museums, and restaurants.
Pre-reunification, Friedrichshain was a largely blue-collar district with strongly left politics (which still hold today). Recent gentrification over the past 20 years has pushed many of the squats out, slowly changing the cultural fabric.