I have an Altura Urban and Ortlieb roll-top panniers.
For everyday use I prefer the flip-top opening of the ALtura, waterproofing has not been an issue in 5 years of daily commuting. This is important as stuff can just be put in rather than having to wrap everything up to make sure it stays dry and panniers take a hard time.
A secure quick release mechanism is really important – the convenience of just pushing a button and lifting, or just lifting in the case of the ortliebs is very useful too. Also an organiser flap / pockets to keep things like keys / phone / wallet handy, and sections to segragate foul weather geart / locks / tools into different sections would be good. Being able to just remove the pannier and take the whole thing with you, as opposed to leaving it on the bike and emptying it is very useful, otherwise pack everything in another bag to take out of the pannier if you’re going to leave it on the bike.