As an Essendon supporter, I'll nominate Collingwood in 1990 as the last pure premiership win.
No lead up tribunal nonsense, no salary cap questions, no behind play hits, no drugs, just a coach who aspired to the romantic notion of leading Collingwood to an enventual premiership.
May have forgotten something that happened during 1990 but which premiership really does stands without questionable conduct or ethics.
2005 Hall suspension, 2004 good win but the coach post match tainted it, 2003,2002, 2001 salary cap concession questions, 2000 Chirs Grant's goal, 1999 Kennett making Essendon footballers vote on prelim final day, 1998 1997 Adelaide being supported by Adelaide supporters, 1996 reasonably pure, 1995 salary cap, 1994 silly finals system (applies to a few other years), 1993 Silvagni touched it, 1992 Ben Cousins barracked for them, 1991 not played at the MCG