I am seeing online that there are a lot of people who state Aroldis Chapman is the hardest throwing pitcher, and I am also seeing a lot of people who state that Nolan Ryan holds that record. Both with some staggering numbers, which is true?
It all depends on how you measure stuff. It's generally accepted that MLB's current tracking system (PITCHf/x) is the most accurate system available, and Chapman holds the record for a PITCHf/x measured pitch with his 105 mph pitch on 2010-09-24 for the Reds vs the Padres.
Various people have attempted to translate older measurements into the same standards as used by PITCHf/x, notably the fact that PITCHf/x measures the speed of the pitch 50 feet from home plate. For example, eFastball estimate that Ryan's pitch on 1974-20-08, which was measured at 100.9 mph at a point 10 feet from home plate would have been travelling at 108.1 mph at the PITCHf/x measurement point of 50 feet from home plate, and thus is faster than Chapman's pitch. But this is an estimate, not a measurement so should be taken with a (small) grain of salt.
It seems that Aroldis broke his own record a few weeks ago (Check that up here!).
It's worth it that Aroldis Chapman is by far the pitcher with most 100+ ptiches in 2015 and since 2010.