The first platform that I wanted to mention in this article is Mintos. Mintos is currently the largest Peer-to-Peer lending platform in Europe, and offer a huge amount of loans to invest in. This makes it a really interesting platform to invest in, as it gives you exposure to several countries and several kinds of loans in each country. That's instant diversification on a single platform. The platform also offers solid returns to investors, around 12%.
On top of that, the interface is really easy to use, and they also have a great portfolio manager so you can set everything once and then just let it invest for you. A large portion of their loans is actually also protected by a buyback guarantee, meaning that even if somebody stopped paying back a loan you will still get your money back.
As for all platforms in this article, I've been investing personally on Mintos (since 2015). Here is a graph showing my accumulated gains from Mintos since I started investing:

As you can tell, I got solid returns from Mintos since I started investing more than 4 years ago. I currently have a solid yield on the platform at nearly 12%, and my money is safe as I invested only in loans with buyback guarantee. Therefore, I definitely recommend Mintos as the best European Peer-to-Peer lending platform. It's the ideal platform to start an investment portfolio or to add to an already existing portfolio.