Rack and panniers obviously, but specifically panniers you can remove and re-attach in a hurry.

I found it very frustrating to have pannier bags that were attached with these long velcro straps that went between the two bags, it made just moving the bike around a lot more difficult than it aught to have been when I had a bunch of groceries onboard or whatever. Now, obviously being able to detach the bags hasn’t made the ride back from the shops any easier, but hauling the lot up a few flights of stairs has become dramatically less unpleasant.

A cycle cape to keep the rain off.
Microfibre cloth to dry the seat on rainy days.
Gloves for warm hands and protection.
Spare inner tube & a few tools.

Can’t live without my rack and pannier. The bag is waterproof and pretty much indestructible, and all my stuff goes in there instead of wearing a backpack so I don’t end up with that huge sweat spot on my back.

Fenders and good lights. Not some million lumen garbage, a good ~80-100lux asymmetrical headlight so that you don’t blind everyone and a wide taillight with 180 degree viewing so you are visible from the side and never run it on a flash or strobe at night.

I am also a huge fan of a front rack with a basket so I can keep my eyes on cargo, and not need specialized bags to carry odd shaped items.

A good multitool and chain breaker can save your ride.

• Rack and panniers
• Fenders for rainy days, some rain gear (I like my cheapo cape) stashed inside a pannier in case you get caught out in unexpected weather
• A small repair kit. Should include a bike multitool, tire levers, a spare tube and pump at the minimum. A chain tool and quick link can be good for chain repairs, but in many years I never had a chain break on me. Mostly flats or random screws getting loose
• Lights for sure if you commute in the dark

Front and back cameras.