Hard to say which is best, there are so many. The highway pegs and backrest are near the top along with the Wheeldock center stand. The FL-ViED's did a lot for the performance too.
I probably have about $2500.00 in mods and am pretty much done. Engine and exhaust are staying stock except for the de-cat and the FL-ViEDs.
The most important mod that made me able to ride in comfort and flat footed was the super narrow by MCC. Tied for first is my stage one with 3.5" jackpots and the Power Vision tuner. Followed by my Iron Cross Ipod interface..