Judging by room quality, my top two downtown hotels would be 1) Golden Nugget 2) Main St Station. I have stayed at both several times, and their rooms have always been very nice. As for the other downtown hotels, I have had some nice rooms and some crappy ones at the same place on different trips. That said, I don't choose a hotel in Vegas based on room quality...I go for the cheapest (or comped) room every time. Give me a bed and shower and I'm happy lol.
Like you, I love the Plaza. Have spent more nights there over the years than anywhere else. But it really went downhill after Jackie Gaughan sold it. They totally ruined the video poker and blackjack, my two games of choice. To top it off, they got rid of the Omaha Lounge "entertainment" lol. I visited the Plaza in March and they are definitely remodeling the place. Hopefully it will once again be the place to play downtown!
BTW I read on another board the Plaza is taking reservations starting August 1. If so, I might have to take a quick weekend trip and check it out!