LG and HTC are the worst, with the most frequent denial of customer assistance. Samsung and Apple have their faults, but you can typically reach someone helpful when there is an issue.
HTC has some of the worst design decisions I've ever seen. The M8 and M9 each contain approximately 20 pieces of tape on the inside, holding important components together. It takes about 4 hours to replace a screen or charging port, and usually doesn't go back together well.
LG is pretty terrible, as well. Read the horror stories about the G4. The G5 is already exhibiting some of the same flaws, such as half the screen freezing or just showing static, or the device not powering on fully straight out of the box.
Nothing is perfect, and every device has its own faults, but I strongly caution people about dealing with companies like this.