Which is the best at&t phones for texting purpose?

First of all, I would say that you should check out wireless.at&t, click "shop all phones" off to the right. There will be some check boxes to the left, check Quick Messaging, and Slider phones. This will automatically load all of those phones, that you are looking for but still I am giving you info about some best texting phones:

1- The Pantech Ease:

The Pantech Ease features a full keyboard, it has2-megapixel camera, Bluetooth (2.0), support for micro SD memory expansion cards (up to 32GB) and a talk time battery life of 300 minutes. And the Texting-specific features are Full Keyboard QWERTY.

2- The Samsung Captivate: no doubt that it’s a nice phone, it sports a full keyboard, a 5-megapixel camera, Bluetooth , support for microSD memory expansion cards (Up to 16 GB) and a talk time battery life of 350 minutes. There is an accelerometer and full QWERTY keyboard.

3- The LG Prime: I am also using LG Prime and it comes with Bluetooth, a full keyboard, a SAR rating of 0.84 W/kg, a memory of 48MB, support for micro SD memory expansion cards and its talk time is 240 minutes. And about texting, I must tell you that it has full QWERTY keyboard so you can enjoy texting as I do. Now choice is yours that which one you want to buy.