For me personally, I had the T-Mobile G1 right at launch, with Android 1.0! It had a hilarious bug that was promptly patched with a quick Android 1.1 rollout, whereby terminal was silently running in the background with SU privileges, which meant that at any time, you could literally type the word reboot on the physical keyboard and the phone would reboot
I was pretty attached to “stock” Android from jump, so when HTC started rolling out their “HTC Sense” skin, I wasn’t a fan. I imported a T-Mobile G2 from the States, which was the same hardware as what was the HTC Desire Z here in the UK, but ran stock Android instead of Sense.
But then in 2011, Samsung released the Galaxy S2. In terms of pure spec, it was just so much more powerful than other phones on the market. It both cured me of my addiction to physical keyboards, and (temporarily) of my preference for stock Android (until I realised Samsung were slow with updates!)
It was pretty clear at that time that other OEMs would struggle since Samsung have always had the advantage of making all the components themselves.