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.

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.

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.

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.

• 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

A good multitool and chain breaker can save your ride.

Front and back cameras.