Custom form, webform, or entityform Would it be best to create my own form or use contrib. drupal7 modules to complete my following need. If i use contrib. modules, which ones should i use.

I want to make a form that has 3 fields. 2 autofill and 1 auto complete. One of the autofill fields would just autofill the current users 'first and last name.' The auto complete field will auto complete existing users in the site - so if you begin to type in a users name it will auto complete for you. The other autofill field will autofill the autocompleted user's supervisor.

Spent a bunch of time trying to configure webforms or entityforms, then kinda wondered if this should just be done with an in house custom form. Whats the best Drupal7 practice for this?

At the end it will email 3 people and then be put into some review process. The 3 people will be the autocompleted user, the autofilled supervisor and a static HR email address. I suppose i would bake that all in a submit handler, if i did my own custom form.
The best and quick way to go with is using EntityForm. The module provides advantage of using field using which you can have Entity/User Reference fields with auto-complete, without writing a single line of code. To notify 3 users on Email, you can use Rules Module. And the other advantage is, it is maintained by Community so all security issues are tracked and fixed before the site can get affected.

Webform module doesn't support auto-complete by default you'll need to use Webform Autocomplete module for that. I've not used this module, so not sure if this provides the option for referencing existing entity.

If you're building it custom, you'll need to build everything by your own using Form API and Mail System. Which will be time consuming as compared to using EntityForm.