for me there is a spectrum of offensive abilities to defensive and consistency.
Offensive/catching fire_____________________________________________________________________________Defensive/Consistent
......Nishikori..........Safin...........................Agassi...........................Djokovic................Murray...................Medvedev
..........Nalbandian.......Gulbis.............................Davydenko
............Benoit Paire
This was done hastily and makes no claims as to whose is the 'best'. It's my opinion and probably reflects a mindset/overall style of a player more so than their isolated backhand capabilities.
To be clear, if I had to select a backhand for myself (rank them) I would have to do so in the context of their overall game - eg. you can't separate Djok's amazing open-stance defensive end-range backhand from a regular backhand, it's all part of the same package. So for me I rank them as follows (I haven't included connors as I have not seen enough of him).
1 - Djok - this is pretty clear for me - day in day out the most rock solid backhand in history. An absorbing wall of rubber - can change direction on a whim, defuse offensive serves and shots, incredible end range, very handy drop shot disguise off this side too.
2 - Agassi - again - can't seperate it from his overall style of taking it early, taking time away, constant pressure. I can still remember watching him live v Schuettler at the AO and was amazed how much of his body he put into that shot on every ball. Crushed.
3 - Murray - let's remember that Murray has some of the all-time great results with clear holes in his game - poor second serve, non ATG forehand. His return of serve stats are off the charts, incredible anticipation. Great variety and feel off the backhand side, can really mix it up well. Obviously can inject pace too. Right up there and only a hair between him and Djok.
4 - Safin - No one before or since has wielded the Prestige like it was a spoon and crushed two'ies. Was the rock of his game when everything else could fall off shallow tracks. Two parts swashbuckling three parts Fort Knox.
5 - Nishikori - honestly I think massively under-rated. Probably the best of this era in terms of sheer ability to change direction, create angle, inject pace. Had a little more pop than Djoker's but less overall consistency and end range ability.
6 - Nalbandian - could easily be higher but I think of Bandicoot's 2'y similar to that of Nishikori. Both smaller guys who could really take it early and punch well above their weight. Great disguise and pace injection. Had the angle, feel, etc. Technically my favourite to model in terms of grip, take back efficiency.