The official Twitter client. Honestly, if you're reading this question, you're probably looking for alternatives to this, but it's a good benchmark to measure other Twitter clients against.
Ranked in these QuestionsQuestion Ranking
Pro Gets the latest API updates
Twitter is slightly infamous for making changes to the API. At least with the official client, you stay ahead of this.
Pro Clean & Functional UI design
Pro Officially sponsored
So you know it will always be supported and won't be randomly shut down.
Pro App experience similar to web experience
If you're switching from using the web client, this app will be quickly understandable to you.
Pro Inline photos
Pro Multiple account support
Allows easy switching of accounts withing the app.
Pro Supports login verification
The official Twitter app is perhaps the only app to support two factor authentication or login verification, thus providing a necessary additional layer of security to your Twitter accounts
Pro Photo editing tools
The app has filters, automatic level balancing and ability to crop built in the app.
Con "In case you missed" feature, showing tweets other users liked in your timeline
Features (can't be turned off) that disrupt chronological timeline and/or show sames tweets repeatedly.
Con Promoted tweets
Con Too many permissions
Asking a lot more permissions than most third party apps.
Con No widget
Recently took away the widget for whatever reason and that's not cool.
Con Pretty heavy
If you're on a "available-space" budget, the official twitter app is pretty bulky and you'll be better-off with a more lightweight alternative.
Con Lack of ability to view tweets in a proper timeline
Con Terrible cellular data management
If you have a limited data plan for your phone, don't be surprised if you find a whopping 800 MB consumption at the end of the month. The app really, really wants to download absolutely everything, even if you don't want to see or play anything.