Disabling buttons on forms
I see a few things that can be done to tweak this:


Cache your selectors instead of re-querying the DOM each time. Anything you use more than once should be cached.  
Since all of the form fields use ID's you can use the native getElementById for faster performance. 
Use strict equality when your are evaluating to reduce the chance of a gotcha. 
Use 'use strict;' if you aren't already.
Surround your code by an IIFE, to encapsulate it and create a private scope.
Namespace your code


`

(function($){
  'use strict';
  var validateForm = {};

  //shortcut functions to simplify code below
  function isEmpty(str) { return str === ''; }
  function isNotSelected(num) { return num === 0; }
  function id(str) { return document.getElementById(str).value+''; }
  function getNum(str) { return document.getElementById(str).value-0; }

  function onSubmit( buttonId ) {
    var disableButton = false;
    var formType = getNum('formType');

    //cache selectors below
    var contact= id('contact');
    var newAccount =  id('newAccount');
    //assuming this is a dropdown
    var el = document.getElementById('select2-entityId');
    var entityId = el.options[ el.selectedIndex ].value -0;

    switch( formType ) {
       case 1:
        if ( isEmpty(id('CreateEntityName')) &amp;&amp; isEmpty(id('newAccountGroup')) ) {
          disableButton = true;
        }
        break;

       case 2:
        if ( isEmpty(newAccount) &amp;&amp; isEmpty(id('lastName')) ) {
          disableButton = true;
        }
        break;

       case 3:
        if ( isNotSelected(entityId) &amp;&amp;  isEmpty(contact) ) {
          disableButton = true;
        }
        break;

       case 4:
        if ( isNotSelected(entityId) &amp;&amp; isEmpty(contact) &amp;&amp; isEmpty(id('need')) &amp;&amp; isNotSelected(getNum('candidates')) ) {
          disableButton = true;
        }
        break;

       case 5:
        if ( isEmpty(newAccount) &amp;&amp; isEmpty(id('country')) ) {
          disableButton = true;
        }
        break;
    }

    if ( disableButton ) {
      $('#' + buttonId + '').prop('disabled', true);
    }

    return disableButton;
  }

  validateForm.init = onSubmit;
  window.validateForm = validateForm;

  //document.ready if you need to do something else
  $(function(){
    if ( validateForm.init() ) {
      //do something here
    }  
  });

})( jQuery );


I am sure there are other things that can be done.  Hope that helps!