I'm not really sure I can answer this one... at least in even relatively short compass. As I first encountered Bradbury when I was 6 (making it 49 years ago), via S is for Space (with the wonderful "Chrysalis" as an opener, followed by the even more evocative "Pillar of Fire") and have, over the years, collected nearly all of his work, including a copy of his first collection, Dark Carnival (which is certainly one of my favorites)*, Bradbury's effect on me is so pervasive that I'm not sure I can be even approximately objective.
Granted, there are some collections which I find to be not as strong as others, such as The Small Assassin... but in that particular case, I think it is the arrangement rather than the stories themselves. As a whole, it doesn't work for me as well, though the individual pieces are another thing. I love The Martian Chronicles and The Illustrated Man; S is for Space (though I am somewhat less taken with R is for Rocket -- darned if I could say why); unlike J-Sun, I've always been very taken with I Sing the Body Electric, which is a rather eclectic collection really; The Halloween Tree remains a strong favorite; The Golden Apples of the Sun and A Medicine for Melancholy (also published together as Twice 22), and The Machineries of Joy... even some of his collections of plays, particularly The Anthem Sprinters and Pillar of Fire and Other Plays; for that matter, his poetry collections, Where Elephants Last in the Dooryard Bloomed, The Haunted Computer and the Android Pope, and Where Robot Mice and Robot Men Run 'Round in Robot Towns; all are quite good.
Some of his later collections are uneven -- at least what I've read, which hasn't been as much as I'd like -- but even there some of the stories are first rate....
*As is The October Country; though, for those who may have run into the frequently repeated comment that one is a reprint of the other, let me assure you that this is not the case. There is a fair amount shared between the two, but there are also a fair number of differences.