I'm a bit old fashioned but, I personally don't like the stretched 737s too much. In my opinion, I think a 737 has to be a short nice little plane.
I think stretching aircraft is good, but to a certain extent. I think the 737-800 is okay, a bit too long for me, but not bad. The 737-900 is WWWAAAYYYY too long in my opinion. The 737-600 is a bit too short, so you guessed it, my favorite 737NG is the 737-700.
I think its the PERFECT length, and perfect size. When I was young, I was around 737-200s and 737-300s all the time, and grown to love them. I used to fly on them so frequently, and fell in love with them. Then all of a sudden when I saw the 737-400, I was a little shocked. For most of my life I was used to seeing a 737 that is short, not a long one. Although, I don't mind the 737-400/800 size anymore, I've been getting used to it. But I don't think I will EVER get used to the 737-900. Its just not a 737.
Hehe, thats how I am, when I get used to seeing something in perticular, I don't like sudden change.
Just a personal preference. They are all great aircraft and wouldn't mind on flying on all of them. I'm just talking about the looks.
My favorite 737NG is the 737-700:
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