Here is the full list of all 48 MX vs. ATV Reflex achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Place 1st in any Race on All-Time difficulty
Lap an Opponent in a Race
Hold a Wheelie trick for 150 feet or longer
Avoid a Wreck in a Race
Perform any airborne Trick and land successfully
Place 3rd or higher in the Waypoint Series 1 in the MotoCareer
Place 3rd or higher in the Waypoint Series 2 in the MotoCareer
Place 3rd or higher in the Waypoint Series 3 in the MotoCareer
Place 3rd or higher in the National Series 1 in the MotoCareer
Place 3rd or higher in the National Series 2 in the MotoCareer
Place 3rd or higher in the National Series 3 in the MotoCareer
Place 3rd or higher in the Supercross Series 1 in the MotoCareer
Place 3rd or higher in the Supercross Series 2 in the MotoCareer
Place 3rd or higher in the Supercross Series 3 in the MotoCareer
Place 3rd or higher in the Freestyle Series 1 in the MotoCareer
Place 3rd or higher in the Freestyle Series 2 in the MotoCareer
Place 3rd or higher in the Freestyle Series 3 in the MotoCareer
Place 3rd or higher in the Omnicross Series 1 in the MotoCareer
Place 3rd or higher in the Omnicross Series 2 in the MotoCareer
Place 3rd or higher in the Omnicross Series 3 in the MotoCareer
Place 3rd or higher in the Champion Sport Track Series 1 in the MotoCareer
Place 3rd or higher in the Champion Sport Track Series 2 in the MotoCareer
Place 3rd or higher in the Champion Sport Track Series 3 in the MotoCareer
Win all Series in the MotoCareer
Win all Face-Off Challenges in the MotoCareer
Place 1st in all events in the MotoCareer
Earn a Gold medal in any MotoCareer Free Ride Challenge
Earn a Gold medal in all MotoCareer Free Ride Challenges
Complete an Online Playlist match
Place 1st in an Online Playlist match with 11 human opponents
Complete 25 Online Playlist matches
Place 1st in 25 Online Playlist matches
Finish ahead of any opponent with an Experience Level higher than yours in an Online Playlist match
Win a Tag Mini-Game in an Online Playlist match
Win a Snake Mini-Game in an Online Playlist match
Complete MotoSkills 1, 2 and 3
Complete MotoSkills 4, 5 and 6
Perform three consecutive, unique airborne Tricks in one jump
Earn a judge's score of 10.0 in a Freestyle Event
Win a Freestyle Event in the MotoCareer without repeating a trick
Win 10 Holeshots
Pull off three unique tricks in a Race Event
Adjust a Tuning Slider and save the new setting
Complete a Race in a Machine with all body panels still attached to your vehicle
Complete a Race in a Machine with no body panels remaining on your vehicle
Avoid 100 Wrecks
Land a jump distance of 300 feet or greater
Hold a Stoppie trick for 75 feet or longer