It's hard to survey the actual MTU in use on the Internet, however it is possible to observe the TCP Maximum Segment Size (MSS) which maybe found the TCP SYN and STNC packets. It is usually (although the use of MSS clamping can cloud the issue somewhat) a good indicator of interface MTU - for IPv4 one just adds 40 to MSS to get the MTU and for IPv6 add 60. Some recent analysis done by APNIC - from Table 1 in the article - provides a list of the most commonly observed MSS values (from a total of 436,095,954 unique TCP sessions - both IPv4 and IPv6 traffic):
MSS Count Proportion
1460 77,911,532 17.9%
1400 70,233,027 16.1%
1370 47,013,384 10.8%
1452 38,914,006 8.9%
1440 37,209,335 8.5%
1360 29,441,242 6.8%
1412 22,738,989 5.2%
1300 18,363,089 4.2%
1380 16,600,330 3.8%