Willie for sure. The first time I played him I was convinced he was cheating because I didn't see how any player could get 40k points in a round.
Of course I was new and could barely score 15k in a round. Now I know he's just really good and can score much higher than that..
There are quite a few other top shelf players who consistently score high. The names I can think of off the top of my head are:
There are lots of great players on harbinger these are just the tip. There are even some good imp teams out there.