Underdog of the Week is a Flash Portal award given to the submission with the highest difference between the Review score and the popular vote. Like the Review Crew Pick, this is an award which suggests a rather specialist/cult interest, leading to a smaller group of people giving the submission strong support. Again, like the RCP it is often won by a crew movie, where the crew has much more interest in it than the general voters of Newgrounds. It can only be won by a submission with over 30 reviews. This award has nothing to do with the Underdog List, which is a list of musicians on the Audio forums.
Good or Bad?
The award is often seen in a negative light by people who emphasise how it is usually won by people whose submissions are backed by their friends who give it high Reviews, but with a general score which is much lower.
Others see it as a movie which appeals strongly to a certain niche of people and is good enough to get 30 people to bother to review it and give a high score, but one much higher than the general vote. These may not neccesarily be friends etc. but perhaps a category of people the movie was aiming for, such as fans of a subject. Users who don't automatically view the award as bad may complain that it is hijacked by crews etc, setting out to make the review score unrealistically high. Many flash Crew's, such as Clock Crew, consider this award well worth having for publicity and various Flash authors and groups specialise in getting UotW, along with their TotW and RC Picks.
Here is a discussion on the BBS by various people about Underdog, including considering its value.