Out of a long list of suggested master thesis topic, how to decide which one to choose?
Background: while I'm not in mathematics, I have some experience crafting these lists. 

I would not worry too much about it being relevant/new/hip. Usually, thesis topics are chosen so that they have at least some opportunity to be analyzed in a novel way (they peeked the interest of the advisor). I would therefore advise against choosing your own topic, unless you really know what you're doing and what the state-of-the-research is in said topic. 

It is, however, crucial that you are interested in the topic, because this is what will determine whether you'll have the discipline and energy for persevering. You said it was hard to determine this a priori, but I suggest you at least try to get a faint idea anyway so that you can narrow down the list to up to three topics.

Besides interest, it is also very important that you talk to the respective faculty/advisors, in order to make up your mind about with whom you want to work. This may prove to be an even more important point since their support may ultimately be crucial in securing that PhD offer. Additionally, I find that talking to people often allows you to get many clues as to what kind of researcher they are and whether that conforms to who you'd want to be.