It is very hard to define what is the BEST ANH Vader helmet as it depends on what you like.
When it comes to ANH Vader casts what a lot of people struggle to grasp is there was more than one cast taken of the original helmet. They are commonly refered to as the UK and US style helmets.
Each of the two styles of ANH helmet casts differ from each other but they are what they are the same helmet but two different snap shots in time of the original.
The ANH Vader changed on screen due to damage, paint being touched up, tusks being glued back on etc so there is no real definitive screen accurate Vader. The original helmet that started filming was a different helmet to the one that finished filming. The UK style mask certainly looks less cleaned up post production than the US style post production - which in essence makes it more like the helmet at the start of filming.
The eFX legend looks the part from what has been shown. Nobody has one yet so no doubt it will be reviewed and analysed to death when it finally gets into the hands of the fans.
The SL is a fantastic helmet as well (brilliant new photos Thomas :thumbsup).
I also like the TM and VP Vaders as well.
There are a fair few Vaders out there all have their own merits.
Cheers Chris