1-Chain whip, for obvious reasons with cassettes.
(The older equivalent is a bench vice for freewheel removal.)
2-Cassette tool/cartridge BB tool. Nothing else works.
2-Engineer pliers that cut cable, housing, grab cable for installations, and handle cable ends.
3-#5 allen wrench (better as part of a set).
4-Third hand for brake calipers, big time-saver in the long run.
5-Presta valve adapter (pretty handy $1 spend)
6-Bungee cords (with a wall, you don't need a repair stand)
7-Headset/cone/bottom bracket/pedal wrenches. Skinny leverage is good.
8-Tire levers, good ones.
9-A good floor pump with a good nozzle end.
Almost every wrenching session includes at least half of this list.