I think Babylon 5 did a great job with their aliens. The centauri are the only ones that can be confused with humans but they actually address this visual similarity in the story. When you learn of their hidden 'tentacles' it gives this low budget take on alien life a distinctively alien flair.
To make a short story shorter, even the centauri are more alien than anything Star Trek has offered us. Besides the farengi, the farengi are rather creative.