In my opinion, the usability and functionality of the HUD is by far the most important. So you should focus on the in-game UI. It's important that the most crucial information is also the most visible which typically suggests that HP/ammo should be in large font and more towards the center. Not necessarily completely centered, just not tucked away in the corners. Keep in mind that it's also important to be able to see as much of the battlefield as possible. It's a balance you have to strike.
I'm gonna disagree with Flatline on a few things here, but it's all personal preference. I think having boxes behind important number fields like HP/ammo increases readability by providing a dark background for you to read them on. And while TanLight is not very pretty, it doesn't strain your eye as much as pure white would.
I also like to have some team-coloured background on the screen to remind you what team you're on in case you forget. In my HUD I have a few thin lines along the edge of the HP/ammo background but I'm seriously thinking of getting rid of those. What I'll stick with is having the team-coloured background of the playermodel in the bottom left corner. It doesn't have to be very obstructive. I just like it there to identify what team I'm on.
Also, a black background with a team-coloured stripe could make the information more consistently readable instead of alternating between red/blue backgrounds. That could make for inconsistent reading. Not that it's likely to make a huge difference, but it might be worth keeping in mind.
These are just my opinions. I'm by no means a big authority on this.
Oh yeah, fuck those default borders with the off-white edge. Screw those. Get rid of them whenever possible (if you also don't like them, it's up to you of course).
EDIT: Animations also help convey important game state info like overheal/low HP/Uber charged/hitmarker etc. Those are important (hitmarker is very much optional). Some are annoying like the spy disguise silhouette or the intelligence pickup one. You might want to get rid of those.