Just a single mod? That's tough!
I like my skid plate (I may be biased though

) because every time I ride dirt I can hear it doing it's job!
Stock tires got changed out before I even picked up the bike and without more dirt worthy tires I wouldn't even be riding where I do!
Bar risers were a good upgrade too. So much more comfy standing!
HTCA's power controller was also a great upgrade!
Hyperpro springs up front were also a big improvement! Wish I had the $$ to go all the way front and rear!
Am I over the limit yet?
Dave