As someone who lives here, and has seen the majority of shows on the strip (good, bad and ugly!), I'd say it's almost a draw between those two, with O getting a slight edge. Mystere (TI) is the grandaddy of Vegas Cirq. shows, but has a LOT of kids at the performance. O (Bellagio) includes water in the performance, KA (MGM) uses a unique 90 degree moving stage, La Reve is not really Cirq, but follows the same theme and uses a huge pool in the performace (O on steriods...), Zumanity is the adult version (R rated) of Cirq shows and includes a flying midget. Love (Mirage) is the newest and uses a Beatles theme. It sounds corny but they pull it off quite well.

Overall, they follow the same theme of acrobatic dance and performance and lots of flying, and while none of them are boring, at $100 plus per ticket you need to prioritize. I'd say KA, Zumanity, Love, O, Mystere, La Reve in that order.

Just my 2 cents...