While Big O appears to be a generic giant robot show at first glance, it distinguishes itself by its quirky characters, old-school artwork, and transcendent music.
The show is a mix of mystery and action with traces of comedy, horror, and romance. The first season is mostly a standard monster-of-the day affair: light on story, high on action. The second season, on the other hand, features an overarching plot. The story is simple, but it serves as an excellent foundation for the show's true strengths.
While the characters are somewhat two-dimensional, they pull off their roles with vigor and panache. Roger, a hybrid of James Bond and Batman, is a mature, suave, and highly likable lead character, a refreshing change from the whiny brats of many recent shows. Dorothy, his android companion, is, of course, stoic, but this makes her frequent flashes of humanity, especially her wonderfully dry sense of humor, all the more precious. The supporting cast is likewise colorful and varied.
The designs are EXTREMELY hit or miss. I myself was initially repulsed by the show's clunky robots. When I returned to the show many years later, I found myself positively enthralled by the old-school artwork. Who needs beam saber wielding fairy-bots when you can have gloriously outdated hunks of scrap metal with rusty anchors and big-a__ piston arms? Not only does the sheer clumsiness impart an air of supreme masculinity, it also adds weight and authenticity to the robots' every punch, collision, and yes, the occasional beam. The art style is also superb, employing the stark lighting and color-blocking found in the classic Batman TV shows. No sparkling manga eyes, gravity defying hairstyles, or doll faces here. Just gritty film noir. This is the reason for Big O's low ratings in Japan and high ratings abroad.
If myanimelist.net allowed a 100/10, this show's music would deserve it. Besides the Queen-style opening, which I hate to this day, this show's soundtrack is hands-down the best I've EVER heard in an anime. Tracks like "Stand a Chance" pump up the audience like the way only classic superhero battle music can. Tracks like "Brick Ballades" capture the weary, run-down glamour of Paradigm City. Tracks like "Stoning" (and "Name of God," a variation of the former) convey a magnificent, almost divine, ferocity. It is the perfect accompaniment to Roger's relentless pursuit of the truth. These two tracks in particular are so exquisite that they could stand next to Mozart's Requiem without shame. The ending theme "And Forever" is admittedly incongruous with the show's dark tone, but its gentle harmonies and charming simplicity set it apart from electro-synth, J-pop garbage like ClariS. In other words, Big O's music is quite simply the best.
If I had counted music as 100/10 (which I really should have), this anime would average out to a perfect score, but that would be just plain silly, so a 9 out of 10 it is.