Is it possible to have all content types as dropdown field in one content type in drupal 7?
I can suggest four different solutions:


Using Bundle Name Reference module , of course I haven't used this module and I'm not sure if it presents the same functionality as you desire.
Easy but not always working , Create a taxonomy vocabulary and add terms having same name as your content type does , then you can add a taxonomy reference to your content type targeting that vocab.
The problem in solution 2 is that when a new content type is added you manually must add a term for that in your vocab , to overcome that problem you can write a simple module then implement hook_node_type_insert and there you can add term to your targeting vocab programmatically so you wont need to manually add new terms any more.
The last and the most complicated solution , maybe you can write it as a module so others can use it in future. You will need to write a module to implement a new field type , you will need to implement hook_field_schema to add new table for your field type (there you can store bundle ID of selected content type) , a field widget which must be Select type and its options can be filled using this function : node_type_get_types.