I don't subscribe to the opinion that there is a "best" way to play rugby at all, I think positive play will stop you from being upset by worse teams but a bit of "negative" rugby will beat teams at your level. New Zealand are leading the way right now with their 15 man running rugby but they only play that when they can and certainly didn't for the majority of the match in Dublin and in Lions tests two and three which saw them struggling at times because they were met by teams who gave them no freedom at the breakdown while maintaining a wide blitz defence with a 9 and 15 who covered the backfield well enough so they couldn't utilise the chip and chase; they didn't play attacking rugby here because when they tried to against such a system that was functioning they got whacked in Chicago. They won one of these four games because the opposition couldn't get past their own ferocious defence, they lost the possession battle 66-34 an territory 70-30 whereas and only ran 100 more metres than Ireland (~320-~420) whereas in the Chicago test possession and territory were equal and they ran over 500 metres compared to less than 200 from Ireland. We saw it again against the Lions that when their backs were against the wall NZ were happy to give the opposition the ball and strangle their attack while trying to force a mistake, its the exact type of "negative" rugby that their opposition can utilise to beat them if executed well, the difference being that its much harder to execute v NZ than it is for NZ to do to anyone else.
I'd argue that the aforementioned defence first style is a more effective way to win a game against teams of equal skill level or teams slightly better than yours, what makes NZ great with their 15 man running game is that it makes it almost impossible for teams who shouldn't really be beating you to get close because any bit of disorganisation will result in desperate defending that will tire a team out or result in a score, this is why NZ seem to always have a game wrapped up by halftime or run away with it at halftime. On the other end of this spectrum I'll take the worst of what I'll call rugby's "elite" (Read as NZ or those capable of beating them) sides, which I believe to be Ireland. Ireland beat New Zealand and lost to Scotland and Wales within the space of six months, the reason being that, in the matches they lost, they couldn't run away with games or leave teams tired enough to capitalise on mistakes towards the end of the game. If they could introduce an effective 15 man running game that would certainly help but if they look to their elite neighbours there's also evidence that they don't need to take that approach, England's front five aren't world class ball players (Itoje probably will be before long) they are four imposing, world class athletes and Dan Cole who can cause opposition packs all world's of trouble and allow their two fly halves get the ball out to their electric 13 and back three, who again aren't world class ball players, to cause havoc with their running game. The unanswered question with them is whether or not they can beat NZ and it'd be a stupid assumption to make that they couldn't when two inferior sides in Ireland and the Lions could. So I certainly think it's possible to be the best without NZ's seemingly unstructured "total rugby" mayhem by going the other way and being a supremely structured outfit and having the players to execute whatever style you choose, and what I currently see outside of my three "elite" sides is that they're all trying to get a balance between the two rather than picking one or the other and perfecting it, or in NZ's case attempting to perfect both, they're mistakes came when they were frugal on D while trying to play like the Globetrotters with the ball v Lions and then sticking to their running game when it wasn't working in Chicago.