The plane I have the most trouble with in a Yak is the Fw 190A-5, because of its ability to quickly change direction via banking, ie split ess. However, once the fight gets on the deck the Focke Wulf is more approachable, since the Yak does scissors quite well too.
I don't have trouble with the P-51D, P-47, Fw 190A-8, and for the most part, the Bf 109G-10. A Yak can defeat the 109 in a rolling split ess contest, given similar E levels. None of these planes can shake a Yak off its tail, given similar pilot levels.
Planes that cause me to concentrate are the Spitfire IX, P-38 and N1K2, because of their good E-retention coupled with very good turn rates. While the Yak has superior E-characteristics to any of these three aircraft, it cannot conduct a sustained turnfight with them. Thus, fights with the P-38, Spit, and Niki are usually strict vertical affairs. However, a Yak will pull away from any of these three, given equal energy.
I have no trouble shooting down Zeros, since it's E-retention is so poor. The Zeke may turn very well, but its turn rate is quite slow, allowing for good snapshot opportunities.