My personal choice is Doctrine 2 for a few reasons:
Conceptually, it's very easy to understand. Working with entities and the entity manager is simple. I love how entities don't need to extend any base model class (like in Doctrine 1).
It's sort of the standard ORM for Symfony2 right now. While it may not be officially 'endorsed' by Symfony2, it's what most people are using, and therefore there's a wealth of information for it online. The value of this is tremendous.
You mention that you've heard "bad things about Doctrine2" - it's important that you realize no ORM is going to be perfect. I've run into some problems with it, but as long as you think creatively, you can find a solution. Sometimes that solution means writing raw SQL, no matter what ORM purists might say. I haven't run into any situation where I regretted using Doctrine 2.
I spent a lot of time looking at Propel, but I just didn't like how it worked. That's not to say it's a poorly programmed library, but for me personally it wasn't a good fit. But since every developer is different, the best advice I can offer is that you look at the documentation and usage examples for both libraries. Since you're going to be the one using it, and not us, you need to pick the library you think you'll benefit the most from.
