user109: In referencing the following article, it is stated that one way to efficiently learn new openings is to study masters that were experts at the particular opening you want to study.  Part of the article states:

Say, you want to learn the Najdorf Sicilian for Black, then get the
  games played by Kasparov who was the biggest expert there.  Looking to
  play the Petroff?  Then Vladimir Kramnik is your man!

So what other masters were considered experts in certain openings?
I will limit the question to specific openings:

Grunfeld
English 
Ruy Lopez  
Scilian  
Kings Indian
Queens Indian
Nimzo Indian
French
Pirc
Caro-Kann
Catalan


user885: Black (1.d4):

King's Indian: Fischer, Radjabov, Kasparov, Gufeld
Queen's Indian: Anand, Karpov, Carlsen
Grunfeld: Svidler, Ftacnik
QGD, Slav: Wang Yue, Sokolov
QGD, Semi-Slav: Shirov, Anand
Nimzo Indian: Kramnik, Ivanchuk
QGD, Classical: Petrosian, Karpov

After typing this, I realized that any answer covering all the common openings played at top level would be ridiculously long. I think you can make things easier for yourself by consulting chessgames.com whenever you want to see top practitioners of any particular opening. For example, typing in "Sicilian, Najdorf" in the search box will give you this page, where you can see which top level players employ it the most. Of course, this doesn't mean that they are the authority on said openings. But, given that the list will almost always include strong GMs (2650+), you can be assured that studying their games in said openings will allow you to learn a lot about those openings, which I assume, is the point of you asking the question in the first place.
Another way of going about this is looking at the player profiles of some of the strongest players of all time (like World Champions, and World Championship contenders), and looking at their opening repertoire. 
In the end, "expert" is a very loose term in your question. So really, you'd be safe in assuming that some of the strongest GMs (and some IMs as well, such as Nezhmetdinov) of all time are/were experts in whichever openings they employed the most.

109: <p>In referencing the following <a href="http://www.chess.com/article/view/the-most-efficient-way-to-learn-openings" rel="nofollow">article</a>, it is stated that one way to efficiently learn new openings is to study masters that were experts at the particular opening you want to study.  Part of the article states:</p>

<blockquote>
  <p>Say, you want to learn the Najdorf Sicilian for Black, then get the
  games played by Kasparov who was the biggest expert there.  Looking to
  play the Petroff?  Then Vladimir Kramnik is your man!</p>
</blockquote>

<p>So what other masters were considered experts in certain openings?</p>

<p>I will limit the question to specific openings:</p>

<ul>
<li>Grunfeld</li>
<li>English </li>
<li>Ruy Lopez  </li>
<li>Scilian  </li>
<li>Kings Indian</li>
<li>Queens Indian</li>
<li>Nimzo Indian</li>
<li>French</li>
<li>Pirc</li>
<li>Caro-Kann</li>
<li>Catalan</li>
</ul>
 885: <p>Black (1.d4):</p>

<ol>
<li><p>King's Indian: Fischer, Radjabov, Kasparov, Gufeld</p></li>
<li><p>Queen's Indian: Anand, Karpov, Carlsen</p></li>
<li><p>Grunfeld: Svidler, Ftacnik</p></li>
<li><p>QGD, Slav: Wang Yue, Sokolov</p></li>
<li><p>QGD, Semi-Slav: Shirov, Anand</p></li>
<li><p>Nimzo Indian: Kramnik, Ivanchuk</p></li>
<li><p>QGD, Classical: Petrosian, Karpov</p></li>
</ol>

<p>After typing this, I realized that any answer covering all the common openings played at top level would be ridiculously long. I think you can make things easier for yourself by consulting <a href="http://www.chessgames.com" rel="nofollow">chessgames.com</a> whenever you want to see top practitioners of any particular opening. For example, typing in "Sicilian, Najdorf" in the search box will give you <a href="http://www.chessgames.com/perl/chessopening?eco=B90" rel="nofollow">this page</a>, where you can see which top level players employ it the most. Of course, this doesn't mean that they are the authority on said openings. But, given that the list will almost always include strong GMs (2650+), you can be assured that studying their games in said openings will allow you to learn a lot about those openings, which I assume, is the point of you asking the question in the first place.</p>

<p>Another way of going about this is looking at the player profiles of some of the strongest players of all time (like World Champions, and World Championship contenders), and looking at their opening repertoire. </p>

<p>In the end, "expert" is a very loose term in your question. So really, you'd be safe in assuming that some of the strongest GMs (and some IMs as well, such as Nezhmetdinov) of all time <em>are/were</em> experts in whichever openings they employed the most.</p>
