Here are all the PvPs that were over 1M buys. The 3 names that standout the most are: McGregor, Lesnar, Rousey.
UFC 229 Khabib vs. McGregor 2018.10.06 2,400,000
UFC 202 Diaz vs. McGregor 2 2016.08.20 1,600,000
UFC 196 McGregor vs. Diaz 2016.03.05 1,317,000
UFC 205 Alvarez vs. McGregor 2016.11.12 1,300,000
UFC 100 2009.07.11 1,300,000
UFC 194 Aldo vs. McGregor 2015.12.12 1,200,000
UFC 116 Lesnar vs Carwin 2010.07.03 1,160,000
UFC 193 Rousey vs. Holm 2015.11.14 1,100,000
UFC 207 Nunes vs. Rousey 2016.12.30 1,100,000
UFC 92 The Ultimate 2008 2008.12.27 1,050,000
UFC 114 Jackson vs Evans 2010.05.29 1,050,000
UFC 121 Lesnar vs. Velasquez 2010.10.23 1,050,000
UFC 168 Weidman vs. Silva 2 2013.12.28 1,025,000
UFC 91 Couture vs Lesnar 2008.11.15 1,010,000
UFC 200 Tate vs. Nunes 2016.07.09 1,009,000