Tau, Eldar, Space Marines and Daemons.
Tau seem to occupy a spot purely because of how crazy hardcore Riptides are. I've had a very inexperienced Tau player annihilate my Chaos Marines (admittedly not a super hardcore list, but not soft by any means) essentially with 2 Riptides. I was able to quickly shred most of his infantry and vehicles to bits, and he TOTALLY FORGOT to use his 10 Broadside suits one turn, then thought charging Crisis suits into my Lord's retinue of Terminators would be a good idea. But those two Riptides tore me apart. I didn't have enough firepower to kill them outright, and they were too fast to engage in combat. Even if I made it in they're T6 with 5 (I think) wounds, a 2+ save, potential for a 3+ invulnerable save and 5+ Feel No Pain. All for significantly less points than my usual Daemon Prince setup (which I didn't field that game actually).
Eldar are nasty. They CAN dominate the Psychic phase but some of their other big tricks are: Massed Wraithguard with D weapons, Jetbikes with Scatterlasers (ALL of them can have Scatterlasers) as Troops, and the ever dreaded Wraithknight. I've only faced Eldar a few times in 7th, and they DO have some nice builds that don't rely on the broken stuff but there is such a disparity between the performance of "good" units and the "overpowered" ones that I've rarely seen them.
Space Marines have a few tricks. I've been on the receiving end of Grav spam on bikes, MSU "free transport" Gladius, and Smash-F*er Deathstar. None were particularly fun, but I found the Free Transports particularly grating. My army spent 200ish points on Transports and this guy got his for free? Hell even if he didn't take transports he'd be 200pts ahead of me in firepower but to have an extra 400ish points just because he took a formation? Not a fan.
I've never seen Daemons do well personally, but I see a lot of internet moaning about it. I might just be thick, but I don't see why they're so powerful. Yeah they can summon more Daemons... but so can I? What's the deal? Invisible Screamers? Cool, I can have invisible Spawn with a beatstick Chaos Lord in the mix. I don't know, I can't wrap my head around it. Whenever I see Daemons pop up in the top 3 rankings of a tournament I just think "why didn't you just shoot them?". But people complain about them, so there must be something I'm missing.