All of them. 

Actually don't get EXPonential Growth or The Golden Gaffe. One if for getting lots of XP and the other is for getting lots of gold. 

None of them. :I

The game sucks. Give it to me and I'll take the damage for you, Heather.

Lol.

Depends. Most of them are only interesting if you've played other Fire Emblem games (Champions, Lost Bloodlines, Smash and Rogues). I'd go for the "The Future Past" pack, because it adds a bit of story if I could only choose one pack. The "Golden" pack (as I think it's called) is basically providing you with easy money and experience, but some say that it's necessary should you want to beat the game on Lunatic(+). The Scramble pack is interesting as well, since it provides a lot of conversations between the characters in the game, adding depth to them. The Challenge pack and the Apetheosis DLC are interesting if you're, well, up for a challenge. They are quite hard actually.
So yeah, each DLC has it's worth, and in the end it depends on how much you like the game and how much you are into the series. If you're a series newcomer, you can probably skip the first batch of DLC I mentioned, and Challenge and Apotheosis might be a bit overkill as well. Again, the Golden pack is only interesting for easy grinding, so you can probably skip that too, which leaves us with the Future Past and the Scramble pack. They're probably the ones I would recommend the most, but again, only if you want to learn more about the characters in the game. If you're not interested in that, you can skip DLC altogether.

Edit: Yes, DLC adds maps and characters. The characters have no actual character though and are only additional units to deploy. Maps are often based off of maps out of older Fire Emblem games. If you just want more to play, you can take any DLC really.

The character-unlock DLC (the first ones I mentioned) add dialogue with characters from past Fire Emblem games, but again, that's only interesting if you know about these characters to begin with.

While I haven't try any of the DLCs I heard "The Future Past" series is the best, everything else should be avoided. 

Some of the maps are duplicated, actually, and the new characters are sorely lacking compared to the ones in the main game (zero supports, for example). A few do interesting things, but ultimately you're paying $2.50+ for a single map, when the $40 game comes with, what, fifty of them? Bad bang-for-buck ratio, in my opinion.

OH MY CATS.  Your lack of culture disturbs me.  You English people ...

Heathers was a brilliantly dark film that should be required viewing for internet access.

I do not quibble over trivialities with the female breed.  As far as I know, you're a huge Beckham fan and like terriers.  You think Mr. Kahn will protect you just because you have a 3DS now?

Ha.  Ha!  Sure, I might be sick and talking out of my head (what does that even mean) but that doesn't mean that ... uh.  I've lost my train of thought.  

Not all of them, but you'll often find that a set of three DLC shares the exact same map, just with different enemies. The early DLC was worse about this.