I don't visit the Flash portal as much as I used to, but I love how Newgrounds is home to almost all of the old classics that got stolen and big on YouTube. Charlie the Unicorn, Numa Numa, The Ultimate Showdown, etc. Newgrounds has tons of content ranging from experimental projects to excellent games and animations.
The art portal is also nice, being a 'more masculine' version of DeviantArt, if that makes sense. Everything from hentai to pixel art to 3D modeling, I've really come to realize how much sheer talent Newgrounds has, although I don't really frequent the art portal too much.
The audio portal might be my favorite aspect of the site. Hundreds of thousands of songs created by independent artists, all downloadable for free and easy to feature in games without much of a copyright issue or even having to take up memory in the actual game file. You could spend forever listening to all of the stuff we have here, and the fact that it's all free just boggles my mind. Newgrounds has the only audio portal on the entire internet; it's definitely Newgrounds' most unique aspect.
Finally we have the forums. A fair balance between active and intelligent, the forums can get bogged down with shit a lot but there are also some really brilliant users here, both funny and helpful.
Overall the audio portal is probably my favorite part.