Moto is probably the worst of them all. Look at the Droid update. Took forever. I didn't follow the Droid as much, but the Milestone got 2.1 around the same time, and we have a few ridiculous bugs like the alarm not working and we are the alarm, not the phone, the music playing when headphones unplugged (yeah that's great in meetings). Worst of all, the Droid isn't smooth yet. I can bet you Moto will never address these issues till 2.2 which I'll just throw out a date like September. Yeah I'll be happy when September rolls around and I get my update. Yes, you can argue that there's faster devices now like the Incredible and the N1, but are you telling me it takes a full 1ghz to run your homescreen smoothly?
Even the Eris runs quite well on 2.1, and so do HTC devices on old ARM11 processors. The question is why a 550 mHz Cortex A8 can't handle Android properly. You can fault Google somewhat, but at the same time Motorola really does nothing to the OS, doesn't optimize it, doesn't really address major issues, and just slaps it on a device and calls it a done deal. If they can't even get a Google Experience phone correctly, what makes you think they'll make their MotoBlur devices well? Shrug.
I don't mind HTC's sense devices taking a while to get updates, but at least the phones come out well. The Legend, and Desire are incredibly smooth compared to similar devices (Droid and N1). I don't mind waiting a bit for a polished phone with a nice Sense UI. On the other hand I do mind waiting for Moto to release its updates which are more of catchup releases and fail at the same time.