There really isn't a tier list but some legions definitely got more love in their design (all of the legion rules are in the same book), especially as the campaign books released which added onto existing rules and introduced some new legion specific units and war gear. Thousand Sons and Iron Warriors are probably the more competitive armies, but all armies are viable.
The Imperial Fist for example hit on a 2+ when shooting bolter weapons (bolt pistols, bolt guns, heavy bolters, etc) which is great because everything comes standard with a bolt weapon. Sons of Horus on the other hand can hit with shooting on a 2+ with pistols, assault guns, and rapid fire weapons if the models they're shooting at are within 12". That sounds good, however if you're within 12" of an enemy unit with your bolters, you may as well use Fury of the Legion (available to all armies, but unable to be used with this Sons of Horus rule) instead and get to shoot all your bolters twice (throwing 4 dice per bolter). A 10 man squad shooting 40 dice at 3+ as opposed to 20 dice at 2+ is going to statistically generate more hits. So its moot save for special weapons like plasma, melta or volkite weapons. So a Sons of Horus player will probably forgo that special rule and use the Fury of the Legion rule instead. Now the Imperial Fist on the other hand can hit with a 2+ with Fury of the Legion since they're special rule works all the time but the Sons of Horus doesn't work with Fury of the Legion.
That however doesn't make the Fists better than the Sons of Horus, they both just play to their strengths. Imperial Fists will more than likely have more bolt weapons, where as Sons of Horus will aim towards having more special weapons. Another Sons of Horus rule is if they outnumber their opponent in a close combat, they get an extra hit for each model that has already fought prior to the Initiative 1 step. So Sons of Horus want to get in really close, blast their guns at their opponent to thin their ranks and then charge into close combat to outnumber them and overwhelm them with attacks. Naturally that means Sons of Horus may want transports, large units, or drop pod deep strikes to do this to mitigate possible incoming fire.