Well, Huzuni is the most known one so it's probably going to win regardless of which ones may be better.
Like @Jess said, Huzuni is by far the most well know client of the list. It is easy to use and popular, so it is often regarded as the "best" of public clients. In my opinion, it is if you're going to pvp in normal circumstances. Majority of well know pvpers on factions such as myself and @moonknight10 use Huzuni. I would suggest a different client for Pvpfest though, as it's fly isn't the best.
Iridium definitely, you can fly fast with it and it has an amazing Killaura, better than huzuni, so buy that.
Iridium is $10 and huzuni is free however, if you set your fly speed to 0.9 on iridium and use /sp speed, you can fly really fast and it a really good client and easy to use once you get used to it.
Calm down, no one is actually "Hacking", the term as it's used here refers more generally to the use of a modified client. Essentially, a modified client is just a different client used to play the same game, but it has different abilities. It involves no actual hacking of any sort, people just call them "hacks" out of misunderstanding, ignorance, and/or convenience. Using a modified client is not the same thing as hacking into a server. The hacking that you are referring to is if someone were to bypass security systems to break into or take over a computer or network, usually to steal information or just as an attack on some entity.