best app to paste in stock text responses

Instead of a hotkey, you can type say "ABC" and have it substituted or expanded into other text. Sort of like a shortcut in the text itself.

You can use System Preferences > Language & Text > Text substitution and set up custom autoexpanders such that if you type "\rtfm" it can replace it with paragraphs of text.

The grand-daddy of autoexpanders is TextExpander app, but its pricey.

If you really need a hotkey or keyboard shortcut, you can roll your own:
(1) Create an automator service which takes no input & just has one action "Get Specified Text". Set the text to whatever you want.
(2) Assign a shortcut to the automator action using System Preferences > Keyboard
(3) Select any text input area, hit the shortcut and the boilerplate text will be pasted.