I'll echo the Ortlieb pannier recommendation. Also the Racktime brand racks, which are the lower cost brand from Tubus. I have the Add-it model on two bikes along with a pair of QL3 office panniers. Not cheap, but they are built like tanks and hold plenty. I also have one of the Ortlieb QL3 Vario backpacks that easily swaps between backpack and pannier modes. I rarely use the latter for commutes, but it still finds occasional use when I need to walk a bit, or need to tote the work laptop around. I did have to purchase another QL3 adapter to have both racks ready to carry, so a bit like Festool if you know the tool brand, that once you're hooked... For the most part my purchases were clearanced or returned items, so if you are patient you can outfit yourself for not too much.

Anyway, the office panniers serve 95% of my needs. On Mondays I usually pack in five shirts and maybe a pair or two of cleaned and pressed pants to cover me for the week. I keep extra pants on hand just in case. During warmer months, I usually only need both panniers on Monday, with one able to hold my lunch, electronics, underwear, and towel for the work shower. I also use a frame pack (Revelate Designs Tangle model?) that mounts to the top tube and uses the main frame triangle. It holds a CO2 pump, tube, patch kit, multi tool, trauma kit, locking chain, and keys. I found the frame bag to be perfect for holding items always needed, so whatever the bag I don't find myself without the basics. During the colder months, I use both panniers for all the other needed garb. I keep toiletries at work.

I am lucky at work that it is a secure campus with covered outdoor bike racks, so I can leave my frame bag attached. It is so secure that plenty of other riders leave lights, electronics, and bikes unlocked. I'm not that trusting, but it is nice to know.

When I do use a backpack I'm partial to the Osprey Manta model. Comes with a bladder and drink line, which is nice. It barely gets any use for commuting, mostly being used for family rides. In my neck of the woods it gets too hot for a backpack and I also don't like the way it throws off my balance when weighted down. Otherwise, the Manta is my go to pack for non-bike travel.