Here are an interesting 1st-week BO collection reports of biggies that released in the recent past since January 2015. This is not a comparison article, but a statistical report of movies that enjoyed great collections in their opening weekend at the Chennai city box office between Jan 2015 - October 2017 in different scenarios of ticket pricing.
 

Before June 2017 (Before GST)
I - 3.83 Cr (531 shows - 5-day opening weekend)
Kabali - 3.49 Cr (942 shows - 3-day opening weekend)
Baahubali 2 - 3.24 Cr (828 shows - 3-day opening weekend)
Vedalam - 3.23 Cr (348 shows - 6-day opening weekend)
Bairavaa - 3.09 Cr (510 shows - 4-day opening weekend)
Theri - 3.06 Cr (456 shows - 4-day opening weekend)

Between July 2017 & September 2017 (After inclusion of GST)
Vivegam - 5.22 Cr (870 shows - 4-day opening weekend)

After Oct 2017 (Hiked Ticket Price + GST + Entertainment tax)
Mersal - 6.86 Cr (678 - 5-day opening weekend)
 

Looking at these figures, I, Vivegam, and Mersal stand as the landmark films to have collected the highest at the Chennai BO in different scenarios. Mersal's collection will now be a new benchmark for any upcoming Tamil biggie to compare its opening weekend collections to set new records.

However, there's a slight dip in the show counts of Mersal, compared to the recent biggies Baahubali 2 and Vivegam, because of the Multiplex strike (PVR & Inox) against double taxation.