PlayStation 4 has surpassed the original PlayStation's sales to reach that #2 spot but it's highly unlikely it will become the biggest ever.
Sony's latest financial report noted that PlayStation 4 had sold 102.8 million units, meaning it had surpassed the original which sold 102.49 million units during its time.
Considering we are in the twilight of PlayStation 4's generation, it's safe to assume the difference won't grow too much but Sony are predicting that a total of 13.5 million units will be sold during the current fiscal year. It's worth noting that the original prediction was 15 million units but that was corrected with the latest report.
A drop in sales is not too alarming since the new generation of consoles will launch in late 2020. Still, the sales not growing means the fabled number of 155 million copies sold is far out of Sony's reach and PlayStation 2 will remain the king of consoles as it's still over 50 million copies ahead of anything else.
On the other hand, during the days of PlayStation 2, online features were rudimentary and nowhere near the capabilities of PS4. New features open up new monetisation avenues and PS Plus subscriptions are one of them. These when compared to the same period last year.
Subscriptions also represent recurring revenue which was not really a thing during PS2 era so it is hard to gauge how much more successful PS2 was than PS4, if at all. All things considered, the era of PS4 may be coming to a close but it has given Sony a period of opulence by any metric imaginable, with the console being the second best selling ever, barring the handheld ones.