It makes sense in most pop-culture sci-fi to make aliens humanoid. Easier to understand, film, animate, etc. In reality, though, the odds of aliens being anything like us is...well, who knows?
I once came up with an alien race that were more like giant slimy spiders, with a big mouth on top and lots of tentacles. Imagine a walking venus flytrap with several dozen Doc Ock tentacles with a body the size of a Hummer, and you're pretty close. The point was, I wanted something so absolutely nonhuman that it seemed really alien. And it was the villainous race, so making them seem like horror monsters also helped.
Trying to think through all the sci-fi I've witnessed, the most alien race I've seen would have to be...the giant floating glowing Jello-looking jellyfish from Mass Effect. The hanar, I believe?
Every other race in the ME universe didn't feel alien. Asari? No way. Turian? Nope. Drell? Krogan? Salarian? Volus? Quarian? Batarian? Vorcha? Sorry. The most alien to me were the Geth, because they thought differently than humans. All the other just felt like quirky humans. The geth (and, of course, Legion) were so fascinating because they operated so differently from humans. I really hope Legion comes back for ME3.
Wow, that got out of hand.....