Restricting it to overseas players I've seen 'in the flesh' playing for British club sides and in no particular order ...

Brett Kenny
Peter Sterling
Wally Lewis
Mal Meninga
Kurt Sorenson
Brian Bevan
Tom van Vollenhoven
Gene Miles
Noel Cleal
Kevin Tamati

Henry Paul
Jamie Lyon
Joe Vagana
Stacey Jones
Adrian Lam
Lesley Vainakolo
Brett Dallas
David Furner
Adrian Vowles
Robbie Paul.

A slight Bradford bias, but i've chosen overseas signings, not just players. People that have done a lot for their clubs.

I know. I was really looking forward to seeing him play but obviously I couldn't say he was best I saw at such a short stint. Thought he was one of the best classy centres I've seen though when he played SOO and for Aussies tourists

steve carter
julien oneil
Kurt Sorensen
Dale Shearer
Emosi Koloto
Esene Faimalo
Robert Relf
noel cleal
kevin tamati
david peachy

Suprised no ones put Jason Smith,was with us for 4 years,the 1st 2 seasons he was simply superb....single-handely destroyed Leeds and Gave a master-class on loose forward play at Wigan when we drew there,his last 2 seasons was wracked by injuries but a truly superb Ball playing 13.

in no order and with a slight biased to my own club:

Les Boyd
Kevin Tamati
Andrew Johns
Allan Langer
Phil Blake
Jamie Lyon
Mal Meninga
Tuigamala (spl?)
Kevin Iro
Henry Paul