I think the two of us have been in an arm wrestle for it since 00 if we accept that as the year Scottish rugby took a dive after a strong 90s.
There's definitely an argument, and a strong one for Wales internationally, two world cup semi finals and two more grand slams. Conversely our test win rate is far far better and we're ranked higher more often than not. Ireland are a very inconsistent team, look at 2018 to 2019 or the 2016 autumn series to the 2017 6 Nations and its cost us silverware for sure. Wales on the other hand generally go well for a while, followed by a weaker period and have churned out as many bad teams as they have good teams in the past 20 years, I feel like Ireland are generally middling to good with 07-08 and 2013 the exceptions where we were pretty poor.
It's obviously a fairly arbitrary ***le but as a 6n power you'd probably back Wales (and I still would, they'll be better than us again at some point in the next 4 years) but given our far superior record against the SH sides I think we shade it internationally in the past 20 years. I think a Wales fan would give up a grand slam to beat NZ, we've done it three times and reached point a slam would be a bigger achievement for us now.