Tires are the obvious choice. It doesn't matter how much you tune your suspension if your running some crap oem tires on the bike your not going to be able to do much. Grippy, decent tires are the foundation to build a bike around. The suspension is important as well, but (stocksuspension+goodtires)>(customsuspension+crap tires) anyday. I'd prefer both but if I could only pick one I'd be ordering up some new rubber.

 

I should have put "for me" after "obvious choice" heh. These bikes come from factory with pretty much everything any of us would ever need for street riding already installed. This pretty much boils down to an opinion piece and could just as well be entitled "Which unecessary mod boosts your confidence the most?".

For me at least, especially with the horrid roads around here I'm always paranoid and paying close attention to traction, so sticky tires, whether needed or not boost my riding coinfidence massively, even though I might never have pushed the stock ones to their limits on the street the fact that these are stickier and have higher limits make me happy.

Pretty much the same thing can be said for suspension, or brakes or pretty much anything else. Whatever makes us feel confident and safe while riding will naturally lead us to push the bikes some and ride better overall, since the bikes come stock capable of going well past our limits then it really just boils down to a confidence thing.