depends on your cassette and your strength / style (mash or spin) but standard seems to be 42 on an 11:28 and go up or down from there.

38 with 11-32 cassette here

I race on 40t, with 11-32 on the back. Works really well. 32 is useful on the back in the recent clag fest, gets you past a surprising amount of people on the steep sticky ups.

Edit – I use a superstar chainring with a 1×10 Shimano set up, with the rear mech spring tension mod. I don’t use any other guide and haven’t dropped a chain yet, despite all the clag this season.

I use a 38t superstar ring with a 11-32 slx cassette, 5701 rear mech and no tension mod or device and haven’t lost a chain yet.

28 at the back sounds a bit short for general riding around, think most people would appreciate 32 or 34 there if you’re running 40, say, upfront.
Like you’ll spank it up any short sharp hill regardless, but you want to be able to sedately proceed on occasion, without feeling like it’s an effort.

39-34-11 here

I can ride everything round here off road in a 28-11 [ dont fancy some of the lakeland climbs though on that gearing] but it feels like SS on a narrow barred bike/lots of standing climbs.

Depends how hilly it is and how strong you are.

Gnarrrr, a Big Un’.

Or a 36, maybe down to 34 but we don’t know what yer cassette ratios are and the Prairie you ride on, you said hills, how steep?

Just fitted a 38t (Superstar) with 11-32 nine speed cassette (MTB mech).

I had a 34t before that which was handy on climbs, but a bit spinny for flats/roads… If I find climbing is becoming a struggle I might consider a bigger cassette maybe, we’ll see…