The only two/three armies i dislike to both play against and/or as are as follows:
Necrons: I HATE how they are so difficult to kill, especially when a Monolith is around, the ability to revive a squad when theres only 1 guy left by teleporting to the Monolith is a pain in the arse. Not only that, but all Gauss shots glance on a 6, meaning my AV 14 Leman Russ's are always going to be hit since most Necron squads that have been fielded are 12-15 strong! The whole res-orb thing ticks me of even more and the phase out rule is always something to not go for if you cant. (Will BE back rolls KILL the army for being so damn hard!)
I guess id never play as them since theres little to zero effort in their building and no character for their armor, they are just killing machines with no souls
Tyranids: I actually have a soft spot for the nids, indeed i dislike playing against them, but I've slowly learn how to properly kill the army without to much trouble. With 3 battle cannon shots, 3 autocannon teams, 1 heavy bolter for my HQ, a hell hound i am pretty well set against their smaller hoard units while worrying little about their big'uns until it becomes a problem. Still, i dislike them cause when they have ALOT of gaunts and Stealers, they are able to easily wipe my infantry grenadiers out and with rending, destroy their support tanks. Wing HT are always a pain since they can flank and are hard to kill
Id like to play as them, they are cool and already ive set about doing a 1000points list, a nice change to be a charging cc army instead of a shooting
Sisters: Okay, i don't TOTALLY dislike them, but the fact they have the S and T of SM, and the low cost means they can field more units with relative ease. Flamers and Meltas are a pain and since they can easily be mechanized, its a real downer when Sisters attack the tanks with melta guns and kill it whilst rhinos block LOS
Play them, nah, they are Xenohunters, and my army is always fighting them cause we use Xeno allies