Now days there are thousands of sunglasses shapes out there. From aviators, over Clubmaster to novelty sunglasses. When picking out sunglasses I look for two things. First, I have to like the shape a lot and second the shape must look good on my face, meaning it has to complement my face. If I can't find one of these things I won't buy the glasses. For example, I love the design of Ray Ban's Clubmaster but they look odd on my face so I never bought them. Also, I  buy sunglasses that will match my wardrobe and style (well, this one is pretty obvious but I thought I will mention it). After years of testing I find these are the shapes of sunglasses I like and wear the most. 
Wayfarer was my first sunglasses crush. Since I first bought them I always keep going back to Wayfarer. Every time I go to Ray Ban to try some other shapes I just end up thinking about buying new Wayfarer in colour I don't have.
My second favourite are Slight cat-eye sunglasses (these in the photo are by Diane von Furstenberg). I don't really like how full on cat-eye glasses look on me, but slight cat-eye really makes sunglasses look special. Frames that are too intense can look fake or distract from persons face, and that is the case with cat-eye frames, I think. Slight cat-eye enhances your facial features. Don't get me wrong, I am all for expressing yourself, If cat-eye (or any other frame) is what you like to wear go for it. 
I already mentioned on my blog how much I love 90's sunglasses (mine are Versus by Gianni Versace). Think of Julia Robert's sunglasses in Notting hill or Sebastian's in Cruel intentions. That kind of frames are my new favourite and can look amazing with any outfit. 
The last are big round sunglasses (these in the photo are form Asos). No matter how much I love small round Lennon style sunglasses I can't picture myself wearing them. Good alternative are big round sunglasses with all the charm of round frames but somehow more wearable, at least for me.