Wheels are always the best upgrade, period. A quality wheelset will always transform the ride on any bike. Drivetrain upgrades much less so. The real question here is what are the parts that make up your drivetrain and is any part of your drivetrain giving you issues? And what is your current wheelset comprise of and what are you considering upgrading to. Without more specific info, we can’t really be helpful beyond what i said in my first sentence.