Genuine must haves on my road bike for me:
Spare tube and puncture repair kit (Carry both as its easier to just change the tube - But if that goes tits up too I can repair it.)
A minipump if your far from home, or a c02 thingy
Tire leavers - Noone likes fighting with a tire, might as well use some weaponry to make it a little easier!
A decent multitool.
A decent lock
A decent set of lights for winter (I dont have mine on the bike in summer at all)
I carry all the repair stuff in a saddle bag, and dont go anywhere without them.
Other must haves (aka stuff I want but dont genuinely need to ride) include
Good set of gloves (Dont underestimate a decent set of gloves! Makes riding so much easier on the hands.)
Good set of shorts (Yep - Lycra

)
A good light jersey (I dont use true under layers in cold weather. I wear my cycling leggings with my castelli shorts over, my castelli jersey with a cheap long sleeve jersey jobby under it and waterproofs over it.)
Decent water proofs - Top, bottle, shoe cover.
A good set of stiff shoes with cleats
Bottle cage
Spare batteries for the lights in winter
Number of bikes I own + 1
TT bike
A track bike
Sorry, getting carried away here
