I won't include the others (Sanyo, Sony Ericsson, etc.) because they either don't offer enough models to remain competitive, or they don't offer models in the US.
I've been in wireless since before anybody besides sci-fi nerds used the word android. HTC was the first, but that does not necesarilly mean the be best. I would say at first, their models were of poor quality, but nowadays they offer solid devices. Samsung, in my opinion takes the title of highest quality devices - so far.
I have faith, though that HTC will start to introduce models with 1.5 GHz processors, Super AMOLED screens, 4.5" screens, etc. to remain competitive. I'd gamble that HTC will even get a good share of the tablet market in the near future.