- Agent Phil Coulson: mentions that he is level 8 early in the show, and in the final episodes of S1 of Agents of S.H.I.E.L.D.
is upgraded to Level 10 when he assumes command of S.H.I.E.L.D.
- Agent Victoria Hand: mentions that she is level 8 in the show.
- Deputy Director Hill: is listed on wikia as level 9, but is unsourced
- Director Fury is identified as the only level 10 operative in S01E14 (T.A.H.I.T.I)
- Alexander Pierce is no longer an Agent of S.H.I.E.L.D, but has a Level 10 Clearance Level.
- In Captain America: The Winter Soldier, Pierce and Fury are identified as "Alpha Level" members. Whether or not this is a reflection of their Level 10 Clearance, or something else (like the fact that Fury is the Director and Pierce is on the WSC) is left unclear.
Wikia's logic for Deputy Director Hill makes a fair bit of sense saying that Coulson and Hand are close to the top, with Fury being at a maximum level 10, but this information has not been specified yet.
As to "Who's On
First Top", as @anthonygrist points out in the comments, there is really no "on top" when it comes to clearances.
Two people can have the same clearances but different authority levels. Similarly, two people can have the same authority level but different clearances. The fact Alexander Pierce has Level 10 clearance is proof that clearance level is not quite the same as operational authority within the organization.
The fact that the file folder says it requires "Level 10 Or Above" clearance is not enough to prove that any levels above 10 currently exist.
Consider the following conditional:
if x >= 10 then access_granted = true
else access_granted = false
This does not necessarily mean that x can ever equal 11 or even 10.1. It just means that my code is not specifically looking for an exact value of 10 and only 10.
If for some reason an additional level of clearance was ever added to S.H.I.E.L.D's system, anyone 10 or above would be allowed access without having to go restamp all the files.
It's also entirely possible that something like a standard adjustable stamp was used. They typically have a standard bit of text or two and selectable values as evidenced by this recent publicity photo which appears to have identical stamps aside from the clearance level number.