Seen Mystere (3), O (twice), Ka and Love. (I also know cast members in two of them). In reading the comments one can draw the conclusion you can't go wrong. There is no bad show IMHO but the ranking is debatable and highly personal. My comments:
Mystere: The original (excluding the Mirage tent) and thus the one I have seen the most. Had to take my new wife so she could experience it too. Probably my favorite because of the focus on the human performers, and less staging (very relative statement). As someone heavily involved in sports medicine the raw display of athleticism is phenomenal. If money counts, Mystere is still the cheapest.
O: My wifes favorite. Less land based acrobatics, subsituted with lots of water. Staging spectacular.
Ka: Awesome acrobatics etc.. Plus phenomenal staging. Sometimes think I preferred this over Mystere.
Love: Very different than the other 3. This is primarily a dance show, with some gymnastics/acrobatics. However, it is a dance show like no other. I am not a Beatles fan, though the music is great and used very effectively. IMHO you do not have to be a Beatles fan to enjoy it. My wife loved it and she hardly knows the Beatles music (I guess she does now).
Aside from dancers vs. gymnasts Love stands in contrast to the other 3 shows in that Cirque shows usually have their own "operatic" music scored specifically for them. Very well done, but and obvious contrast to the Beatles music.
Another point, Cirque appeals to all ages, I saw Mystere a 2nd time when I was a single father with 11 & 14 year old teens, they loved it. Saw "O" with them 2 years later, along with their grandparents. Everyone loved it. Saw Love with 14 to 75 year olds in family last year, all enjoyed the show, even though I was the only Beatles generation member.
You can't go wrong no matter which one you pick. Have not seen La Reve .. thats for my next visit.
Randy