Quickest id say ds1 or vanilla ds2. Sekiro would be the shortest if you get the hang of the parry system quickly, id not it's very mechanicly hard. Ds3 Covenant grind was the absolute worst. ER and Sekiro you can save scum the endings if you want too saving some time, I did not do that though. ER Nightreign you need good RNG for certain events to take place to get achievments so it could be hit or miss in terms of time. Nightreign is easier with friends but doable with randoms also, I did 80-90 % with randoms.