Tricky. If Arsenal get past PSV then they have one of the Madrids which isn’t easy by any stretch! Leverkusen seem to have an easier route but got Bayern first so who knows!!
As a Benfica supporter, I'm just happy to be here, what with all the injuries the team has. And if we can manage to actually have a shot at getting past Barcelona, I just hope we don't throw it away this time.
I mostly just enjoy seeing the Liverpool fans having a go at Real Madrid. Having Liverpool fans/half of r/soccer discover how dirty Real Madrid's defence is was cathartic.
In terms of underdogs, seeing Aston Villa reach far would be fun. Based on your username, I'll assume you'd rather have Liverpool reach far though :)