I tried every race at least once but until now my favorites are the Hacan, Creuss, Jol Nar and L1z1x.
I think the Hacan are perfect if you are planning to focus on the political cards and have alliances. You can give another player a trade agreement and get one with a value of 1 or 2 and demand, that they always vote in your favor. I did this with a Creuss player, he gave me his trade agreement with a value of 2 and we agreed that he votes in my favor on every third politcal card. After some time I took away two of his systems and demanded, that he would vote in my favor every second political card, in return I would give him one of the systems back I just conquered.
In this game I also payed the other players to vote in my favor and so you can dominate the political part of the game, simply because you get so much money (we played with the Trade II card and I had trade agreements worth 5 ressources, so I got 8 to 12 trade goods every round). Aditionally I had Mecatol Rex worth 6 influence.
With so much money I was able to afford a technology every round, and I still had enough money to have a big fleet (in the end I had both war suns, all dreadnoughts and many other ships).
The only thing I hate about the Hacan is that they loose a big advantage if you don't play with the Trade I card, because no one has to pay a CC to gain trade goods, so the special ability is worthless.
The Creuss are one of my favorites because of their mobility. As soon as you can place the additional wormholes you can be almost anywhere at anytime. All other players would have to put a destroyer or advanced fighter in every empty system in their inner border to prevent you from just popping in.
And the wormholes allow you to have your production center far away from the enemy systems and your fleets can be there in a single action.
I think that the Creuss are one of the strongest races, since every other has to expect an attack from every possible direction, it puts them into the situation that they should not thin out their fleets but yet they have to protect much more systems. The Creuss can ignore that, because they can have one big fleet and still have the mobility to defend almost every system.
About the Jol Nar I like the fact that, if you have enough resources, you can get so many technologies and, like Saikoro said, steamroll almost every enemy. But they are not my biggest favorites, because they are specialized on a part of the game that is very vulnerable, I mean you need really a lot of resources to get many technologies and if you have bad planet around you you are pretty much f….
The L1z1x are in my list of favorites because they start with good technologies, have a good fleet and have very good abilities. I think they can survive with almost any setup and can easily become one of the biggest threat in the game. The only problem is that experienced players know that and try to hold you down and sabotage you with politcal cards as much as possible.
About all the other races: I have played many of them only once, so I don't want to judge about them but many of them sound very good and I think some of them need some experience, I mean that you have to play three or four times with them in order to really understand them and play well.