While there are certainly cheap plastic hand guards out there, competitively priced drop in rails and Magpul makes a nice replacement for the old M-16 hand guard, I personally will never buy anything that does not allow the barrel to free float.
Considering that, the quick answer on who makes the best rail system for the price is Samson Mfg or Yankee Hill Machine. You can read my review of YHM's SLR rail for more details on it.
If you want the most solid rails money can buy, get the VLTOR MUR monolithic rail or the Mega Arms monolithic rail. You can also check Legion Arms, LaRue, Hogan Guns and Armalite for mono/poly uppers, but I have used the 2 I recommend and can vouch for their strength and machining quality.
If you want the most solid rail that will work on a standard upper, then check Barnes Precision Machine and Daniel Defense.
If weight is a big factor for you, then Troy/VTAC TRX battlerail extreme used to be my first choice. However there are lots of contenders now. I would check Barnes Precision's lightweight rail or Samson, or YHM. I have seen a couple of Troy forends fail. They were awesome about replacing it (free), but the design of pinching barrel nut and flexing the metal is not as strong as their competitors. They (Troy) do have a new system now, but I have not had a chance to see it yet.
But there are a lot of great rails out there and to find the perfect one for you means making sure you know your needs.
Well, looks aside, you should get the longest rail your barrel will allow. It gives you more options for attachments, allows a greater sight radius for iron sights and allows more flexibility with shooting positions and grips. BUT! The downside is weight and for many shooters, every ounce counts, particularly if it is forward of the magwell. Personally, I don’t like too much weight but I deal with it for a longer rail. I try and get rails that are about an inch to an inch and a half shorter than the barrel.
When selecting a longer rail, you need to consider the way it attaches to the upper. Many rails come with their own barrel nut. I have yet to see a system that is not good for a 9" rail or smaller, but if you are looking at 13" or 15"+ of rail, the ability to apply torque to the join between upper and rail increases significantly. Without stress testing and comparing rails, I am limited to what a small amount of engineering has taught me. Troy and YHM (old system, not the SLR) are at the weaker end of acceptable by my reckoning, with Daniel Defense and BPM at the other end. Stronger still, of course is a monolithic/polylithic system. An example of a mid range system for strength at the attachment point is JP Rifles, Samson and YHM's SLR. Again, all the listed brands are perfectly acceptable to me, but I do want more strength if possible. One of the rails I was disappointed with was the old LaRue – Even though I would never mount something over it, I still don’t like the gap in the top rail or the attachment system. The new LaRue rail is nice, but propriatary and for the money, it makes more sense for me to go with Mega or VLTOR.
Finally, you need to decide what you want to mount on the rail and if you want rail the whole way or just a free floating handguard. Being able to bolt on rail makes some of the lighter handguard style offer a little of the best of both worlds. Most companies make some kind of rail free handguard that you can bolt rails onto now.