Daniel Defense - Their LITE rails is the best rail system out there right now. The mounting system is index free which allows you to torque the barrel nut correctly, they're strong, light (lightest in the industry in general) and they have a great profile...more like a standard M4 hanguard, not fat and round. They recently lowered their prices so they're even a better deal.
LaRue - This would be my second choice. Good rail system and the company makes solid product but I don't like the mounting system and it doesn't give you a continuous top rail. Heavier than the DD and has a rounder profile compared to the DD.
Midwest Industries - Cheap price. Fat and round. HEAVY. Some people are into that kinda thing I guess.
POF - Stay away from this rail, it's the last rail I'll install on my AR's. The top rail mounting system isn't very good and it's butt ugly to boot.
SWS - Not a bad rail system but it could use a better mounting system. No anti-rotation and it's no particularly light considering how much open space it has.
Troy/Samson - Easy install, fairly heavy compared to DD and has a rounder profile. If easy install is your main deal, get the DD Omega rail. It's actually EASIER to install.
YHM - Cheap price, rails are not Mil-Spec, crap mounting system...etc. I do use some YHM items (like their Phantoms) but the rail systems aren't so good. Also, their standard rail is SUPER HEAVY. Their "light" rails are also quite a bit heavier than DD offerings.
KAC - Good rail system. Their Free Floats are decent but the mounting system is antiquated (like the DD M4, LaRue, etc...) compared to the DD Lite. Their URX is very nice and it would be second choice over LaRue if you didn't need $500+ in installation tools.
ARMS - I really dislike their SIR rails. Relatively heavy, unnecessary amount of screwing on of various rail sizes, odd "bi-level" rail on some their rails, questionable material choices, etc. Their newest rails (that mount similar to the Troy/Samson) aren't so bad but still not that great. I'd buy a DD Omega or Lite over ARMS any day of the week.
Vltor - CAS-V aren't bad rails but again, I don't like having a elevated top rail. If that doesn't bother you, Vltor makes great kit.
Honestly, if you don't know any better (ignorance really is bliss), any of the rail systems will probably be great. But once you tried them all, you'll figure it out. I tried them all out (meaning I've owned every single brand listed) and found the DD the be the best rail system all around. I've actually expunged all the other rails out of my collection.