Dooku is a very impressive Force-User. Undoubtedly. Plagueis-tier imo? No. I also agree he does not wield Malgus' raw potential. Honestly I do not think there is someone to my knowledge that is comparable to Dooku in how they apply the force. Dooku is very precise in his application, powerful obviously though I feel he is very opportunistic and uses his abilities when they will achieve the greatest impact. He's arguably the anti-thesis of Galen Marek. Wheras Marek is a wrecking-ball of force-energy, Dooku is a finely-tempered blade that strikes where it can achieve results. He is by no means weak and I am not saying he only strikes when it will work because he HAS to to get anything done, just that it is his Modus Operandi. (I believe that is the term anyways)
In any case if I had to rank him...... obviously < Yoda, Sidious.
Imo, definitively below Plagueis, Revan, Kun etc. as well.
However he is obviously > Maul, Ventress.
He is difficult to place in my opinion in who he is NEAR, and much easier, obviously, to just see who he is above and below.
However after working ten hours and dropping a large post in the VS thread which likely led to this one's spawning, I am a bit drained. Apologies if my answer is not "definitive" enough but there it is.
As always, thoughts and opinions are appreciated on anything I say. Unless they are undue-ly harsh or antagonistic.