Scarlets, Ospreys, Newport-Gwent Dragons and Cardiff Blues were joined by Celtic Warriors for their only season in the league, along with the four Irish regions and three Scottish teams.
Another feast of attacking rugby saw 651 tries for an average of 4.9 per match and the Welsh regions were largely dominant throughout, occupying five of the top six league positions by the time all was said and done.
The Early Winners
Having beaten local rivals Leinster 15-8 in their season opener, reigning champions Munster welcomed Llanelli Scarlets to Thomond Park in week two, boasting an eight-year unbeaten record at the famous Limerick ground.
The Scarlets though, had grand plans of their own and in particular, fly-half Gareth Bowen. The Welsh playmaker was exceptional, scoring all 19 points as the newly-formed team won 19-12 to lay down a substantial early-season marker.
Munster's fall from grace continued, losing to Dragons, Ospreys and compatriots Ulster in consecutive weeks.
Leinster too struggled to replicate the form that had seen them lift the inaugural crown two years earlier, but Ulster went from strength to strength in challenging the Welsh powerhouses.
After losing a seven-try thriller to Ospreys on opening night, Alan Solomons' side won eight of their next ten, including a wind-swept 10-9 victory over Scarlets to put them top of the pile at the half-way stage.
Munster did not have the best of seasons after winning the title in Year Two
Run-in:
With Ulster in a dominant position at the league's mid-point, Scarlets' indifferent form continued. Following their loss to Solomons' outfit, they lost twice more in three weeks to Welsh rivals. First falling to Dragons and then the Celtic Warriors at Brewery Field.
Warriors however, were on a title-charge. Welsh legend Neil Jenkins was at his metronomic best, successful with 44 consecutive kicks that earned 118 points for his side.
Remarkably he did not miss a kick at goal from October 2003 until Scarlets visited the home of Bridgend RFC in February 2004.
Though Jenkins' side were victorious, it was to be the turning point in Scarlets' season. The Llanelli side racked up victory after victory in a run that saw them unbeaten for the last nine games of the season, with a 33-33 draw against Connacht the only blot on their copybook.
Ulster meanwhile dented Warriors title charge with a thumping 28-0 victory on Welsh soil, while Dragons continued to accumulate results to put them in the title mix.
Percy Montgomery appeared for a Dragons side that had a fine first year in the Celtic League
A Llanelli side boasting Scott Quinnell, Dwayne Peel, Mike Phillips, Stephen Jones, current Welsh forwards coach Robin McBryde and Irish counterpart Simon Easterby would always be hard to beat, but they owe a favour to Cardiff Blues for a gutsy victory over Ulster that set up an enthralling league finale.
Grand-Slam winner Tom Shanklin scored two tries in Cardiff, while Nicky Robinson and Iestyn Harris ran proceedings for the home side against a dogged Ulster outfit. The Welshmen emerged 19-15 winners to present a serious speed-bump to Solomons' title charge with just four games remaining.
Scarlets and Ulster both won their next three games before an enviable final day clash at a packed Stradey Park.
Final Day:
As the league reached its climax, Dragons had a chance to make history on its final day, but they encountered an inspired Leinster side at the old Lansdowne Road.
In a 12-try extravaganza, Leinster racked up a half-century against the title-challengers, with former Ireland lock Malcolm O'Kelly grabbing an unlikely brace.
With Dragons out of the hunt, all eyes turned to Stradey Park and it was Wales' former record cap holder, Jones - playing his last Scarlets game before leaving for Clermont Auvergne - who came to the fore.
Jones kicked 18 points, with Simon Easterby exceptional in the pack, and it was the stand-off who burst through a gap to put centre Matthew Watkins over for the decisive try.
Ulster did fight back with a late score of their own through Neil McMillan, but Scarlets had done enough to clinch their first and only Pro12 title.
Scarlets' veteran stars failed to reproduce the irresistible form of 2004 the following season, a combination of Jones' departure, an injury to McBryde, and an ageing Scott Quinnell in his final season meant the team's spine was diminished.
Since 2004, Scarlets have finished no higher than fourth - in 2007 and 2013 - even finishing ninth in 2010, but with Wayne Pivac's men currently sitting pretty in third, 2016 could yet be their year.
Ulster had their hearts broken after losing a final day shootout with Scarlets
The Top Scorers
Jenkins' 44 consecutive kicks sees him top the points standings with a remarkable 273 points, closely followed by a young Gavin Henson on 265 and Scarlets' other fly-half Bowen completing the top three on 240.
Honourable mentions to Leinster's Brian O'Meara and Springboks great Percy Montgomery, playing for the Dragons, who finished fourth and fifth respectively.
Cardiff Blues' Jamie Robinson was top try scorer on 12, Conor Phillips, of Connacht, a close second with 11. Jason Forster, Sean Lamont and Tyron Howe all finished with nine scores apiece.
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