Some races seem to work a lot better against others. I was wondering, which ones produce the most even match up?

I think that the people decks fall roughly into the categories of hard-hitting power decks (ie big element powers for some of the character cards), finesse decks (lower power character cards but lots of ways of manipulating the fight in your favour with the support/booster/leadership cards), and a mixture of both.

The first category is probably better to play as an introduction to Blue Moon, and includes peoples such as the Vulca and Terrah.

The second is much harder to win with at first, but after many games become difficult to beat. These include the Flit, Buka, Khind.

The third category of decks are good for a balanced game, eg Hoax, Mimix, Aqua.

Playing like with like will give you a more even game, especially if pitting the intermediate category decks against each other.

Having said that, it's got more to do with the player's skill and hand management than pure deck choice. The game has also been extensively play-tested and the preassembled decks put together to ensure that they are as balanced as possible.

My preferences for interesting games are Pillar vs anyone (can get some insane combo's which anihilate opposition, or can get beaten really quickly), Vulca vs Terrah (big powers, Earth vs Fire, good even game), Khind vs Buka (both tricky, trying to build large values over consecutive rounds whilst limiting your opponent to less than 5 cards on the table). 

 Bairoth - great answer, very helpful.

 I only have the original game, and I was wondering about getting more, that is very cool to know, thanks!

A lot of the match-up even up after a few game when you know what to expect of the other and how to play against. I find every match up interesting now. 

So, what new race do I buy first?