Maingear, DigitalStorm and iBuyPower sell rebranded Clevos/Sagers and MSIs. None of them seems reliable, but I have not bought anything from them.
Razer is OK, but it charges a high price for mobility.
Sagers and MSIs are cheap and usually have lots of customization options and passable build quality. Easy to maintain, service, and upgrade.
Alienware is pricey and comes with Dell's decent warranty and mediocre quality control. Build quality and cooling seem better than the rest. Accessing the cooling system and upgrading CPUs and GPUs is not difficult and does not void the warranty. Dell gives plenty of large discounts.
Look into the ASUS ROG G750 too. Cooling and ergonomics seem good, probably better than Alienware's, but you have to tear apart the system (likely voiding the warranty) to access the cooling system.