For Android devices in general, ASUS has an excellent track record. But if you mean specifically phones, HTC devices are usually more dependable. Motorola phones have a very high rate of failure after short usage, for example, the classic Razr phone had a 35% failure rate (according to friends who worked in an AT&T call center) when it was AT&T's entry level phone. That's not such a big deal if it's just a simple phone that does, but a smartphone that dies can take an awful lot of data with it.
They both have some great devices however I'd day HTC. They tend to stick to higher end devices while Motorola makes both higher and lower and devices. The HTC One is coming out soon and it blows away anything that Motorola has done so far.