You don't have to pay money at all; houdini and fire are both free to download and have ratings of ~3400 ELO
A New Engine

Rybka 4 is coming out in the near future. Of course you must wait for this.
Really??? We gotta wait for the future?
+1 for the laugh
Playing strength is the determining factor and you are worried that Fritz 12 won't be as good as Rybka?
You can set Fritz at 5 seconds/move and it will crush you all day long in under 30 moves.
You'll never know the difference between Fritz, Rybka, Shredder, whatever. They are all WAY too good for anyone that's not rated at least 2400 ELO.
I'm still using Fritz 7, i see no need to get the latest version.

Stockfish 8 is nothing less than awesome. It makes good use of more advanced hardware. Running on my 8 core 4.7 Ghz beast, not only has it not lost it has never had a draw with either white or black
I posted before the release of Stockfish 8. Now the best engines are:
For blitz:
1. Stockfish 8
2. Houdini 5.01
3. Komodo 10.2
For classical:
1. Stockfish 8
2. Komodo 10.2
3. Houdini 5.01

True that, but it is intestine to hear what other people think of the current engines. For instance I think Komodo 10.1 is stronger than 10.2
Well, the last Top Chess Engine Championship was won by Stockfish:
https://www.chessbomb.com/arena/-/2016-tcec9

First, you must decide the reason for purchasing an engine. If it is to play against you, then ANY of the modern engines are fine in that they are all so strong they will crush the typical player. If it is to analyze your games, then an engine that somehow also generates some commentary might be helpful. If it's to play against others on the internet (in venues which allow this), then Rybka 4 (soon to be available) will undoubtedly be the best you can do. But for $50 or so dollars, any of them will beat the pants off you.
I didn't know Giri was already on chess.com 7 years ago!
Way better is
TOBY DEEP TACTICS!!!
TobyTal garenteed to beat Rybka 3 and Fritz 11 in match play with tablebases off!
tobychess.com
What a joke...