Best bag for cycling?

I used a Dakine for years when I commuted, and now use it on my MTB. It's not one of the cycling specific ones, just a "lifestyle" bag, around 18l. Two mesh side pockets, one soft-lined top pocket that's ideal for stashing your phone and wallet for quick access. Mine has survived 7 winters and the inner waterproofing has only just started to break down (possibly more to do with being unused over lockdown and left fully loaded on a hook in the garage through the summer).

What I like about it is that it's really unobtrusive on the bike, even heavily loaded. It's quite narrow and the strap design seems to almost snug it in against your body. Certainly well tucked out of the wind. Will easily carry the spares you want. Not that expensive at the moment either. This is the current equivalent I think - https://www.dakine.com/collections/lifestyle-backpacks-1/products/wonder...

Proviz 30L backpack including a bladder pocket for easier on the go drinking.

https://www.provizsports.com/en-gb/reflect360-cycling-backpack.html

I never leave home without it on a commuter run.   Although Im pretty sure its had a price hike or 2 since I bought one 3 years ago.  Could have sworn it was about £45.

I have a Deuter 15L rucksack. While I like it's slim, low profile shape and it is very comfortable I find it doesn't hold very much gear. Waterproofing is non-existent too.

Some more ideas - https://road.cc/content/buyers-guide/19-best-cycling-rucksacks-219033

It's not a rucksack but how about an oversize saddle bag. I have a 12l bag that I find so much easier to cycle with than carrying a bag on my back. Obviously it depends on how much you want to carry but I was able to carry everything that used to go into my bag on my commute (who remembers those). I used to get a bit of lower back pain on longer rides with a bag but that might just be me.

I'm a very big fan of the courier bag, very good access and some can be massive, but compressable and with the correct straps, stable.  And pouches can be added to the strap, utility belt style.

Timbuk II were a favorite, but their present range seem wanting.  I have yet to decide which bag to get next, I sort of need one.

I'm a massive fan of Osprey rucksacks. I have a 10l raptor and 25l Escapist

I've had them for years. They're pretty bombproof and extraordinarily well designed in terms of utility and comfort. The hydration packs are superb too - the Raptor came equipped with a 3l one, the Escapist is suitably compatible and I just swap it over. I also use them for hiking and running.

The 10 gets used for travelling light, the 25 can carry enough stuff for a day with me and my daughter. They're not cheap, but they are versatile. Really can't fault them.

My Mrs bought me one of these when I asked her to get me a waterproof backpack for commuting etc

https://www.over-board.co.uk/products/classic-waterproof-backpack-30-litres

She took waterproof literally as it is meant for boats etc. However it works well, no leakages in over 18 months including full on, "am-i-under-a-waterfall" deluges. However it is one large compartment and you have to buy a seperate thing to hold laptops, tablets, pens etc. However I can take pretty much all my change of clothes for the week and a clean towel into work on Monday with no hassle. 

Only downside is if you don't roll the top down and do it up in a certain way, the corner can be in your vision when glancing over your shoulder. Slight adjustment and it is fine though. 

Osprey Talon or Escapist. Or perhaps a Carradice saddlebag?

Buying a rucksack for one trip seems like slightly overkill, so I would also consider when else you would use the bag, and how much you want to spend. If you do lots of hiking (for example) then it might be worth getting a bag with the features you want for that, even if it's not perfect for this weekend trip.

If doing trips by bike is likely to be a relatively common in the future, I would also consider a rack + panniers as a preferable option to a rucksack for cycling with.

To answer your question: Osprey are generally highly regarded, and have several cycling-specific models. However, they are on the pricier side and may come with more features than you need. Any decent outdoors shop will have a range of brands and models, so it might be worth just having a browse. I have a Patagonia Refugio (28L) as my everyday rucksack which is fairly simply but I'm very happy with and sounds like about the right size for your needs, although I'm not convinced I'd want to cycle a long way with it. I'd probably look for something with good wide hip straps to get the weight off your shoulders.

I don't have any recommendations for rucksacks (sorry) but just wanted to say please don't put anything in there like keys, etc... that could puncture your skin if you were to fall off. Enjoy the riding!