Which property portal should I use for searching for apartments in Germany?

The German internet portals for property are of course in competition with each other. Each develops new features aimed at both sellers/landlords and at buyers/tenants. And so each has a share of the market and tries to increase that share.

As such no internet portal has a full and comprehensive listing of properties for sale or rent!

Some sellers advertise only using traditional media such as newspapers. Other sellers use both traditional and on-line media. Some sellers advertise though just one portal, such as Immowelt, and others market their apartments through two or more portals.

And different websites have different shares of the market in different regions of Germany.

In Berlin market leader is https://www.immobilienscout24.de

Other major players in berlin are https://www.immonet.de and https://www.immowelt.de

But there are many other property websites out there.

See here an overview of German property portals: https://www.immobilienportale.com/uebersicht-immobilienportale/

A new kid on the block, and increasingly major player, is https://www.ivd24immobilien.de
This is a platform for IVD (Immobilienverband Deutschland) members, who must place their properties on here at least a week before they are listed on any of the other portals. So a lot of good properties can be found on here first.