Cheapest?
Spray painting the radiator shrouds black. They shoulda come that way in the first place.
Runner-up:
Home-made fender eliminator.
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Most useful?
Bar-end mirrors (Napoleons at ~$30 a pop, not cheap, but not expensive)
Runner up?
Sargent Seat. Not cheap at all, but your boys will thank you.
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Best Stealth Mod No One Will Notice But You?
TORS. Looks stock. Runs stronger. Buy 'em used and save.
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Most utterly unnecessary mod offering only modest practical improvement while remaining totally, completely bitchin' to the point where it gets the most comments?
Pazzos. Go figure