In order of most used to least used:
1) Allen wrenches (or at least a 3 way star with 4mm, 5mm, and 6mm)
2) Screwdrivers
3) Cassette tool
4) Crescent wrench
5) Chain whip
6) Chain tool
7) Pedal wrench
8) Cable cutters
9) Scissors
10) Rags
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It's like riding a bicycle