I want to plug in an external HD on it to torrent up never ending (mostly for seeding), with lowest priority. It should even automatically completely stop if it disturbs my net usage, and go back in later.

And I want to be able to give gaming (or my computer) the highest priority, because of its ping requirements. It should be above even of Skype and any voip, which also have high priority.

Web surfing and "youtubing" comes in the middle - it won't matter much if it delays a second or two.

Finally, interface as simple as possible. It's fine if it's command line setup, but it needs to have a GUI.

What are my options for routers / firmwares as described?

Maybe Gargoyle? Or even Time Capsule? It seems to me good old DD-WRT isn't the best option out there anymore. I wonder what else could cover these grounds.