Top ten coolest ways to enter into insert mode

  a - append after cursor
  A - append at end of line
  i - insert before cursor
  I - insert before beginning of the line
  o - open a new line below current line
  O - open a new line above the current line
  s - substitute a character
  r - replace a character
  cc - delete the whole line and enter into insert mode
  C - delete forward of cursor and enter into insert mode

If you want to learn more about inserting text checkout

  :help inserting or
  :help A
  :help cc