Its difficult to compare different playing positions when determining the "best" player as flashier positions like wing are always going to be more glamorous options with this in mind I would go, I'll also be using some stats which I know should be taken with a pinch of salt, given they mean little without context:
Fullbacks:
Honorable mentions: Milner-Skudder, Izzy Folau
My Winner: Ben Smith
While Milner-Skudder is a devastating ball runner Ben Smith has the awareness and kicking game to boot, he's really the complete fullback. Izzy doesn't get the credit he's due this season as he's scored about half the tries but has become an amazing link player to add to his own running, leading the competition in offloads, defenders beaten and second in assists.
Wings:
Honorable mentions: James Lowe, Hougard, Osborne, Naholo
My Winner: Nadolo
Naholo and Nadolo have both been equally devastating for their respective teams despite having different playing styles. Naholo is all about crazy pace and his ability to beat the man one on one, where as Nadolo is more of a battering ram, however Nadolo has developed an amazing offload to supplement his game and their respective stats for defenders beaten/clean breaks and tries are similar. Hard to separate these two but Nemani edges it due to Naholo's hands letting him down with the most handling errors this season.
Centres:
Honorable mentions: Kerevi, Ma'a Nonu
My Winner: Damien de Allende
Kerevi has had a fantastic season but maybe needs a little more time to develop as a player, as he can get lost against some opposition. The main argument here is between Nonu and de Allende, both have been fantastic but I personally feel de Allende has more of an impact for the Stormers, consistently being the star of the show, with a crazy number of defenders beaten, clean breaks and offloads. Nonu has been marginally quieter but is the better defender. I'm giving it to de Allende partly because of my man crush as well as the desire to see at least one Saffer make the back line of this dream team
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Flyhalf:
Honorable mentions: Sopoaga, Foley
My Winner: Barret
A lot of sides have had instability in this area and none of the saffer fly-halves have really performed consistently enough to crack it. Foley and Sopoaga are great players but for his consistent ability to be a game breaker, Barret wins.
Scrumhalf:
Honourable mentions: Nic White, Aaron Smith, Nick Stirzaker, Cobus Reinach, Faf de Klerk
My Winner: TJ Perenara
White and Smith are solid complete scrumhalves, the other three are great young talents with heaps of potential but Perenara wins this one for the same reason Barret did. Consistent ability to break other teams apart.
Maybe I'll post forward opinions later. Perhaps we can have a SR team of the competition thread? Unless it's too early.