It's kind of hard to say "best ad this cycle", since various ads have different purposes. Some are biographical, others are attack ads, others seek to prop up their candidate's favorables, others seek to depress voter turnout, and so on. So an ad that is really good for one thing may be terrible for what a candidate really needs.
But this ad, from Vote Vets, is more relevant and hard hitting than anything else I've seen this cycle:
This ad brings home the "body armor" problem like nothing else.
Vote Vets is also running newspaper ads against Joe Lieberman. Incredible work so far.
Update: People who contribute to air this ad will get to vote on which state a version of this ad should air next.