AssembleMe

square without the stroke - that's a photoshop geek would say anyway.

I would say the neon-green square. Am I wrong?

first red square with frame on the left is odd - there's nothing odd about it
the other ones are either small,green,round or frameless

First red square - it has always one more attribute in common with the shape you're comparing to than the rest of the shapes.

The question did say “shape”, therefore it must be the circle.

Gotta say the first one. It has only one property different from all the others. The first one is the hub of the "similarity graph".