user806: I want to invest my hard-earned money to buy a chess engine which can help me to:

analyze games easily
experiment with different opening repertoire
strategical analysis
positional play

I don't expect an answer like, all are good enough to beat a human player, etc. I am not looking for an engine whom I can beat, as I know it's not humanly possible for a 1600 rated player to beat these engines, but my purposes are different (as given above).
So, if you have or use any of these engines, please let me know a comparative detail on which one I should go for.

user825: The other answers are a bit old... The best answer is Stockfish 5. Stockfish 5 is rated number one on the rating list
http://www.computerchess.org.uk/ccrl/4040/
and it's free!

user2180: Houdini if you want to spend, Stockfish for a free engine.
Houdini is simply the most human-like.

user1582: About a year ago, I chose Houdini. I had read that Houdini excelled at position evaluations while the other engines tried to excel with tactics but seemed inferior with positional concepts. Using Houdini and comparing it with Fritz, I agree that it is positionally stronger. I also observe that Houdini is very strong with tactics. Houdini can quickly find mat in 15 and other things that are beyond my comprehension. But position play and position evaluations are far more important since those concepts often have a longer range impact.

user865: I would suggest you look into some open source engines. And tools for analysis.  These will be free. Some will be quite fully featured and very very powerful: 
Stockfish. Rating: 3121
Critter (free) Rating: 3207
Also, look into SCID: an information database to which you can import millions of games and pair the database with analysis engines, add chess move-engines etc. 
    -https://en.wikipedia.org/wiki/Shane%27s_Chess_Information_Database
note 1: there are millions (literally) of master games which have been annotated, and can be downloaded in zipped .pgn files, then which can be imported to SCID for play through etc. So analysis, openings, endgames etc, can all be analysed via scid though whichever engines you decide on (no reason to only have one). 
note 2: Crafty finished in second place in the 2010 Fifth Annual ACCA Americas' Computer Chess Championships. Crafty lost only one game to the first place winner Thinker.
note 3: In various computer chess rankings Stockfish 2.1.1 was second or third behind the top gratis program Houdini and the free program Critter.
Good Luck!

806: <p>I want to invest my hard-earned money to buy a chess engine which can help me to:</p>

<ul>
<li>analyze games easily</li>
<li>experiment with different opening repertoire</li>
<li>strategical analysis</li>
<li>positional play</li>
</ul>

<p>I don't expect an answer like, all are good enough to beat a human player, etc. <strong>I am not looking for an engine whom I can beat</strong>, as I know it's not humanly possible for a 1600 rated player to beat these engines, but my purposes are different (as given above).</p>

<p>So, if you have or use any of these engines, <strong>please let me know a comparative detail on which one I should go for.</strong></p>
 825: <p>The other answers are a bit old... The best answer is Stockfish 5. Stockfish 5 is rated number one on the rating list</p>

<p><a href="http://www.computerchess.org.uk/ccrl/4040/" rel="nofollow">http://www.computerchess.org.uk/ccrl/4040/</a></p>

<p>and it's free!</p>
 2180: <p>Houdini if you want to spend, Stockfish for a free engine.
Houdini is simply the most human-like.</p>
 1582: <p>About a year ago, I chose Houdini. I had read that Houdini excelled at position evaluations while the other engines tried to excel with tactics but seemed inferior with positional concepts. Using Houdini and comparing it with Fritz, I agree that it is positionally stronger. I also observe that Houdini is very strong with tactics. Houdini can quickly find mat in 15 and other things that are beyond my comprehension. But position play and position evaluations are far more important since those concepts often have a longer range impact.</p>
 865: <p>I would suggest you look into some open source engines. And tools for analysis.  These will be free. Some will be quite fully featured and very very powerful: </p>

<p>Stockfish. Rating: 3121
Critter (free) Rating: 3207</p>

<p>Also, look into SCID: an information database to which you can import millions of games and pair the database with analysis engines, add chess move-engines etc. 
    -<a href="https://en.wikipedia.org/wiki/Shane%27s_Chess_Information_Database" rel="noreferrer">https://en.wikipedia.org/wiki/Shane%27s_Chess_Information_Database</a></p>

<p>note 1: there are millions (literally) of master games which have been annotated, and can be downloaded in zipped .pgn files, then which can be imported to SCID for play through etc. So analysis, openings, endgames etc, can all be analysed via scid though whichever engines you decide on (no reason to only have one). </p>

<p>note 2: Crafty finished in second place in the 2010 Fifth Annual ACCA Americas' Computer Chess Championships. Crafty lost only one game to the first place winner Thinker.</p>

<p>note 3: In various computer chess rankings Stockfish 2.1.1 was second or third behind the top gratis program Houdini and the free program Critter.</p>

<p>Good Luck!</p>
