Each MV I will review will be judge according to different criteria : the aesthetic of the MV, the choreography, the outfits of the girls and the quality of the song. Then I will give a simple score for all these criteria ! 

- The aesthetic of the MV : This MV has got bright colors which is a things I like a LOT and the background of the dance shot is not empty (compared to what we saw in Atarashi Watashi ni Nare or Ee Ka?!).
The beginning, where all the girls are lying on the ground in circle is simple but pretty and the idea of all the electrical wires around them (one even make a heart shape) is cute and as you can see, all of them are in the official colors of the members ! 
Anyway I'm not a huge fan of the close-up : a simple curtain and some pink stuff floating... typical H!P. 
I also really like the moments during the musical bridge where the light goes dark because it really suits the "rock vibe" effect of this part of the song ! 

- The choreography : S/mileage improve a LOT in dance and you can see that in this MV. Even if the dance is not that hard, it's very powerful ! I'm not a huge fan of the dance during the chorus but I'm really into the musical bridge part. 
Anyway I think they need to give S/mileage harder choreography because I think they can totally handle it. 

- The outfits : Classy and pretty, a thing that you can expect from a song about being eighteen (cute outfits would have been childish for example). None of the girls have got a bad outfit compared to what we saw in Otona na no yo! from Berryz Koubou (come on Maasa...). Anyway I'm missing the little "S/mileage emblem" which disappeared since Atarashi Watashi ni Nare but I guess it's to show that the group is no more a group made of children.  

- Quality of the song : If I had to choose between Mystery Night and Eighteen Emotion, I will choose Eighteen Emotion for sure. First, it's a happy song even if the lyrics are not so good. Secondly this song looks more like a S/mileage than Mystery Night and this is the S/mileage I want to see and listen. The musical bridge is great, not a lots of things to say about it. 

SCORE : 7 smile out of 10 !  

THIS IS ONLY MY PERSONAL OPINION AND YOURS CAN BE TOTALLY DIFFERENT.