It depends on how you define, best, of course.
It’s also a great question to ask college students, which I’ll do tomorrow. (And yes, I’ll give extra credit points to anyone from class who comments on this blog, today). If I asked my students in September to find the best Tweet ever, they probably would have cited (but not in APA style) Buzzfeed or the Huffington Post:
–The 19 Most Epic Tweets Ever Tweeted (Buzzfeed)
If only Bradley’s arm was longer. Best photo ever. #oscars pic.twitter.com/C9U5NOtGap
— Ellen DeGeneres (@TheEllenShow) March 3, 2014
–Here Are The 22 Coolest Tweets Ever, According To Twitter (Huffingtonpost)
Four more years. pic.twitter.com/bAJE6Vom
— Barack Obama (@BarackObama) November 7, 2012
What’s best in regard to marketing?
The Tweets above are excellent: not too wordy, great photos, and emotionally satisfying. Both Tweets also communicate the Tweeters’ personal brands. But the Tweets on the HuffPost and Twitter lists were not selected for their branding, but for the powerful way they communicated to fellow Tweeters. However, my favorite brand Tweet is on Twitter’s list. Oreo tells the best brand story and demonstrates marketing agility with this famous Tweet:
Power out? No problem. pic.twitter.com/dnQ7pOgC
— Oreo Cookie (@Oreo) February 4, 2013
For marketing expertise using Twitter, my students should be turning to business sources such as Adweek and Mashable.com.
Hint, hint, here are two good lists:
Baby got back! We like big jets and we cannot lie, this 737 is ready to fly. pic.twitter.com/N9BaWWi3SR
— Delta (@Delta) July 30, 2013
–The 10 Best Marketing Tweets I’ve Ever Seen (Melanie Curtin via LinkedIn)
My favorite on the list is the faux feud between Old Spice and Taco Bell. These are two brands that understand Twitter is about conversing with fans vs. shouting at them.
Taco Bell and Old Spice are well known in the world of tweet-marketing, not only for their own individual brilliance, but the bright light of their combined ingenuity. This exchange, for example, should win some kind of joint prize:
Sara Kerr 
As I had said in class, I really love the Mall of America’s twitter account. However, I enjoy their twitter because they respond back to people and what they are saying about MOA. Sometimes they’ll attach pictures of GIFs to their responses as well, which can make things really entertaining. Another twitter account I really enjoy is Matt Bellassai; he is a writer for BuzzFeed, and he is freakin hilarious. I can relate to him and his tweets on so many levels. In looking through the list on AdWeek, I was extremely surprised not to see the Cold Water Challenge anywhere. Whether you were nominated or not, everyone has heard of the Cold Water Challenge! I would also say it was extremely effective in raising awareness about ALS.
I do not have a Twitter account, but I can understand the satisfaction behind this fast moving social media platform. These tweets are especially engaging because they have a clever and short message and include a picture that is aesthetically pleasing. There is a satisfaction in each tweeted picture: Ellen’s Oscar photo includes several well-known celebrities, Obama’s adorable picture of him hugging Michelle is heart warming and the glow of the Oreo highlights that it is dark, but you do not need power to eat an Oreo.