Komodo is a chess engine known for its excellent positional judgment, including unequal material conditions. One of its programmers Larry Kaufman is an opening expert. He took care that the evaluation of opening variations by the chess engine is in accordance with well-known opening theory. Of course Komodo is also tactically a top engine, it is not the fastest calculator, but it calculates very deep.
Houdini and Rybka
Houdini is the best chess engine at blitz and at seeing tactics quickly. Rybka was a long time the chess engine of choice. Houdini is basically an improved Rybka, so there is no reason to buy this engine anymore.
Deep Fritz and Fritz for Fun
Deep Fritz has been long around and is improved over the years. It is no longer the number one, but it gives original results and is even in some special situations still the best engine. Fritz for Fun is the same engine, but then 32 bit. So a high value engine and a good choice for a small budget.
Fritz (Nalimov) and Syzygy Endgame Tablebases
Endgame Tablebases are complete databases that provide the perfect answers for a set number of pieces in the endgame. This is especially useful for correspondence players, when analyzing the endgame. Fritz (Nalimov) Endgame Tablebases (number 3) has solved all 3, 4 and 5 piece endgames and 12 6-piece endgames. The Syzygy Endgame Tablebases (number 4) has solved all 3, 4 and 5 piece endgames and 27 6-piece endgames. The Endgame Tablebases have to be bought separately.
A Powerbook contains the most relevant openings theory based on over a million high class tournament games. For each opening position all relevant information is stored: all moves that were played in the position, by players of what average rating, with what success and performance results. Giving the opportunity to develop an opening repertoire or let the engine play on the strongest way in the opening. The power books have to be bought separately.
Chessbase Graphical User Interface (GUI)
A good user interface for a chess engine is very important. The Chessbase user interface is the most comprehensive and complete. You can install multiple purchased chess engines under this interface. If you for example you have Stockfish or HIARCS, these can be installed as an UCI engine. Also Powerbooks or Tablebases can be installed under this interface.