These are the 3 most popular tools for me.  They are all located in the tool palette under the Windows menu.

It is much easier to hit their related key instead of going to the tool bar...there are no modifier keys.

The arrow tool, for selecting things

Keyboard shortcut is "a"

The blade tool, for cutting

Keyboard shortcut is "b"

The pen tool, for adding keyframes

Keyboard shortcut is "p"