Most notable Greg Brady/Barry Williams. I heard him sing on that song Former Child Stars in Dickie Roberts Former Child Star, and he sounded terrible. Maureen sounded pretty good though. My sister used to try and compare him to David Cassidy, but as far as I know, David still is a very good singer.

I've never heard the song you speak of here (maybe I did when I saw that movie in the theaters years ago and forgot it) and maybe he did sound bad on it, but I've heard him sing good otherwise.

Barry Williams is a very good singer- he has done a lot of plays like Man of La Mancha plus has his act in Branson. Maureen is a close second. I remember in the 90's she released a country album. She was also in a celebrity singing competition that was on years ago. I forget the name and never actually watched it but it was country music themed.
While she is not a original Brady, Geri Reischl aka Fake Jan had the best voice of them all!
I recently watched some Variety Hour clips and Maureen did a beautiful version of Time In a Bottle- worth looking up on YouTube- I stumbled upon it. Love that song and she did justice to it.

The others made do, and were not terrible. They did the best they could with the voices they had. Carol, on the other hand, sang every chance she got. I bet Florence had meetings with sherwood begging to sing on the show, lol. She sang a lot on TV in the 70's, from what I see on my DVD's of The Love Boat and what I've read on IMDB.

Remember when Greg sang this?

Clowns never laughed before,
Beanstalks never grew,
Ponies never ran before,
till I met you!!!!!!!!

Well, if I recall correctly, Barry and Maureen had both said that it was rather Mike Lookinland who was the most musical of the Brady kids. Though the older two were also good at it.

And from a fan review from the TV.com page about the "Amateur Nite" episode (with "Sunshine Day" and "Keep On"), it sure looks that way:
Let's review their performances, shall we? First, I feel bad and embarrassed for Cindy, Peter, and Jan. Or otherwise: Susan Olsen, Christopher Knight, and Eve Plumb. It's obvious they weren't into this whole thing. They had trouble remembering the choreography. Susan had no rhythm in either of the songs, same with Chris, who I'm pretty sure wasn't singing in either of the songs. And Eve looked like she was in pain towards the end of "Keep On." Now let's move on to the more enthusiastic kids: Marcia, Greg, and Bobby. Or again: Maureen McCormick, Barry Williams, and Mike Lookinland. Maureen had a little trouble in "Keep On." She forgot to hold the microphone up to her mouth for almost the entire song, but otherwise her dancing was usually on key. There's not much to say about Barry. He didn't really mess up, but he didn't stand out, either. I'm shocked he didn't give himself a solo or something. Now Mike, he had me cracking up watching him the entire time! From his "groovy" solo to his "far out" dancing, I think he nailed it. Sure, his singing could have been better, but he was making an effort. I loved how he's probably the most underestimated Brady, but he definitely brought his A game to his performances with his rhythm and style here. Bravo, Bobby (or shall I say Mike?), bravo!

Although, Mike didn't like dancing much, which was why he didn't want to do the variety hour in the first place.
I remember that in Susan's book about the BBVH, his mother had commented on it:
“Michael didn’t want to do it. He’s a real good musician, piano player, guitarist, and singer. But he’s not a dancer and he knew there would be a lot of dancing. It’s hard to dance when you’re not good at it.”