I want to buy my neice a pannier that she can take to school. Her books are SUPER heavy. Suggestions that would fit on her rear rack?
I can't say any of this is "the best" but I have one and it might be right for this application:
Ortlieb Vario Pannier: https://www.rei.com/rei-garage/product/198425/ortlieb-vario-ql21-pa...
Converts from pannier to backpack, not huge but well-structured and supportive. It takes about 2-3 minutes to put the backpack straps on to convert it to a backpack, but it comes on and off the bike very quickly. Pluses: supportive, two straps, waterproof
Minuses: that time-to-switch. Also a little spendy. You'll need to mount the special mounting bracket to the side of your rear rack.
Buy it if: commute is all-weather and it's worth your time on arrival to have the backpack support carrying your stuff around.
What I use on a regular basis is a "regular" Ortlieb pannier (the yellow originalish ones) which I leave on the bike, plus a smallish cloth grocery bag that fits inside it. But I don't have all that much to carry around during the day; I can leave most of my stuff at my desk.
TBH the best other option I've used regularly is a regular ol' backpack inside a grocery pannier or a Wald basket pannier. It loses to the Ortlieb backpack in waterproofness and slickness, but it beats Ortlieb in quickness, also in that if you have extra stuff, you can just wear the backpack and put the extra stuff in the pannier.