Designing URI for current logged in user in REST applications
It's up to you. All the approaches are perfectly fine from a REST perspective.
According to Roy Thomas Fielding's dissertation*, any information that can be named can be a resource:
5.2.1.1 Resources and Resource Identifiers
The key abstraction of information in REST is a resource. Any information that can be named can be a resource: a document or image, a temporal service (e.g. "today's weather in Los Angeles"), a collection of other resources, a non-virtual object (e.g. a person), and so on. In other words, any concept that might be the target of an author's hypertext reference must fit within the definition of a resource. A resource is a conceptual mapping to a set of entities, not the entity that corresponds to the mapping at any particular point in time. [...]
When using /me, /users/me, /users/myself, /users/current and similars, you have a locator for the authenticated user and it will always identify the concept of an authenticated user, regardless of which user is authenticated.
For more flexibility, you also can support /users/{username}.
By the way, a similar situation was addressed in Is using magic (me/self) resource identifiers going against REST principles?
* If you are interested in REST, the chapter 5 of Fielding's dissertation is a must-read.
I think REST URIs should uniquely identify the resource, no matter it' using userId/email/ssn or username, whichever attribute uniquely identify user in your system.
So, resource can be users (plural /users) and to make it singular we have below options,
If client has userId, resource should be something like,
GET - /users/{user-id}
If client doesn't have userId, but has username, then
GET - /users/{username}
So, as long as uri uniquely identifies user, we can use above uri patterns as a REST resource.
If, client doesn't have userId, username or email or any other attribute which uniquely identifies user in your system, then, we can have resource uri something like,
GET- /users/current
OR
GET- /users/me
But, in this case, client needs to have user specific TOKEN or session enabled, so that server can find user from active session or token passed in headers. Note, we should consider this a last option.