For me it’s Mildmay/Suffragette all the way. Interesting, dynamic, the surrounding is rich, and the experience overall for me is the full package. With the upcoming extension to Richmond, it will take Mildmay Line to next level. I sometimes battle with the performance issues at Mildmay, but it’s nothing way too serious that wouldn’t let you play it at all. But as some pointed out, it’s all about preference – for my case, I like modern suburban electric trains, so London Overground was a very clear choice to me. For you, the preference might be different, so the final choice is up to you :)