A search turned up over 400 results.  There are fewer plugins that are tagged “twitter”, but even these amount to 259.  In attempt to find the “best” I extracted details of all 259 and placed them in a spreadsheet.  I then eliminated all those with an average user rating of 4 or less, and those that did not look relevant.  The rest I sorted on the product of average user rating and number of downloads to find the most popular and highly rated twitter plugins.

The top 20 are listed below:

Twitter for Wordpress

Twitter for WordPress displays yours latest tweets in your WordPress blog.
Downloads: 149,778
Average user rating:4.4

Twitter Widget Pro

A widget that properly handles twitter feeds, including parsing @username, #hashtags, and URLs into links. Requires PHP5.
Downloads: 56,705
Average user rating:4.3

TweetMeme Button

Adds the TweetMeme button into your posts and RSS feed.
Downloads: 40,940
Average user rating:4.7

WP to Twitter

Posts a Twitter status update when you update your WordPress blog or post to your blogroll, using the Cligs URL shortening service.
Downloads: 36,992
Average user rating:4.8

Pretty Link

Shrink, track and share any URL on the Internet from your WordPress website. Create short links suitable for Twitter using your own domain name!
Downloads: 35,264
Average user rating:4.3

Tweet This

Adds a  Tweet This Post  link to every post and page. Shortens URLs. Can automatically tweet new blog posts. Highly customizable.
Downloads: 37,701
Average user rating:4.1

Twitme

This plugin allows you to automatically post your new posts on the twitter website.
Downloads: 23,053
Average user rating:4.2

WordTwit

WordTwit is a plugin that utilizes the Twitter API to automatically push a published post to your Twitter account as a tweet.
Downloads: 17,302
Average user rating:4.8

Tweetable

Integrate Twitter with your WordPress blog. Automatically tweet new posts, display your latest tweet in your sidebar, etc.
Downloads: 17,422
Average user rating:4.6

Wickett Twitter Widget

Display tweets from a Twitter account in the sidebar of your blog. As seen on WordPress.com.
Downloads: 12,997
Average user rating:4.7

MyTwitter

MyTwitter allows users to display their recent Twitter updates (tweets) on their blog and update their status through the Options page.
Downloads: 13,555
Average user rating:4.6

Digg Digg

Integrate  Digg Button ,  Reddit Me Button ,  Dzone Button ,  Yahoo Buzz Button  and  TweetMeme (twitter) Button  Into Wordpress Content.
Downloads: 8,989
Average user rating:5

Tweetbacks

Show tweets that mention your post as “tweetbacks” in your comments section.
Downloads: 12,231
Average user rating:4.2

Twitter Friends Widget

Displays your Twitter friends with their profile images in the sidebar.
Downloads: 7,103
Average user rating:5

TwitterCounter

Integrate TwitterCounter.com badges on your blog to display the number of followers you have on Twitter.
Downloads: 6,946
Average user rating:5

Subscription Options

Adds subscription option icons for your RSS Feed URL; your FeedBurner Email Service URL and your Twitter Stream URL. Totally user-defined.
Downloads: 9,249
Average user rating:4.1

@ Reply

This plugin allows you to add Twitter-like @reply links to comments.
Downloads: 7,223
Average user rating:4.5

Twitter Friendly Links

Your very own TinyURL within your OWN domain! If you DO promote your blog posts in Twitter, then you MUST make your links look cool!
Downloads: 8,547
Average user rating:4.1

SimpleTwitter

SimpleTwitter displays the most recent twitter post (tweet) for a given user. The tweet is cached on the webserver for a number of minutes.
Downloads: 6,350
Average user rating:4.7

BackType Connect

The BackType Connect Wordpress plugin lets you show related conversations (from Twitter, Digg, FriendFeed & more) inline with your own comments.
Downloads: 5,331
Average user rating:5