I'd say the leading contenders are:
Nacewa and Rocky Elsom for Leinster.
Rush, ***o and Blair for the Blues.
Howlett for Munster.
Filo Tiatia and Holah for Ospreys.
King for Scarlets.
Out of them, I'd possibly eliminate Elsom (awesome impact, not long enough stay to be considered the best import), ***o (good player, but not great), Holah (went missing in too many important games) and possibly Howlett (great signing for Munster nevertheless). Rush's dissapointing season also puts him out of the running for me, despite the fact that he's been awesome up untill now.
So, Nacewa, Blair, Tiatia or King. Probably slight bias towards imports to Welsh regions, but they're simply the teams I watch the most. It's difficult to choose between them really. Nacewa benefits from being part of a more successful team, but he's been an important part of that success. The other three have been or were big players for their respective teams for a number of years, and have been consistently awesome. I'm gonna go for King though, and I can't give a propper answer as to why, I've simply enjoyed every moment of watching him play.