I've seen O, Love, Viva Elvis, and Believe; my DW has seen these as well as Ka and Mystere.
DW tells me Mystere, being the oldest Cirque show on the strip, is very much "classic" Cirque. Ka has fantastic music (we have the soundtrack), and is supposed to have a very strong, epic story with fantastic acrobatics and costume.
Of the ones, I've seen, I love LOVE the most. As others have mentioned, it's set in an amphitheater where every seat is a great seat. The action on center stage feels like it extends out for a lot of scenes, and scenes like Yellow Submarine and All You Need Is Love literally cover the audience.
Viva Elvis was a bit of a letdown. We saw this after LOVE and had high expectations after we saw what Cirque did with The Beatles. Viva Elvis was set in a traditional theater, albeit very comfortable. But the whole thing was more of a retrospective. Seriously, a narrator floats on stage, talks about Elvis' life and career, and cuts to different acts. It felt very disjointed and the costumes and acrobatics were kind of lacking for a Cirque show.
We heard raves for O, but it was kind of a letdown. It's Cirque with water. Even though there were a lot of beautiful scenes, it felt slow and kind of boring. For a water show, we much preferred Le Reve at the Wynn, produced by former Cirque director Franco Dragone.
Believe was the worst of the lot. We thought, Cirque + Criss Angel + magic + dark fairy tale = amazing. Nope. The costumes and sets were top notch Cirque, but it should not be called a Cirque show. The whole show hinges on Criss Angel, and he's not a likeable performer -- he just isn't -- and we kept rolling our eyes and cringing in our seats every time he popped up to try to impress the audience. Sadly, we paid for our tickets although we saw promoters literally giving away tickets in the Luxor lobby if you would just attend a timeshare presentation (we thought this sort of thing only happened on TV). We had to wonder how many of the audience were attending the show for free while we felt like complete fools.