Which Real Time Clock Modul is best?

DS1307? DS3231? ...? What's your favourite rtc module?

how do you #define best ?

ds3231 has a smaller drift so imho better, also a bit more expensive

but it depends on the accuracy you need for your project, sometimes you want an internet link to an NTP server and sometimes a circadian clock will do.

No NTP connection. It's for a standalone clock. So it should be pretty accurate.

define "So it should be pretty accurate."

that's too vague ...

what kind of error is acceptable 1 second per hour 1 second per day 1 second per year

robtillaart:
#define “So it should be pretty accurate.”

that’s too vague …

what kind of error is acceptable 1 second per hour 1 second per day 1 second per year

1 second per day is acceptable.

DS3231 is your choice - http://tronixstuff.com/2014/12/01/tutorial-using-ds1307-and-ds3231-real-time-clock-modules-with-arduino/ -

Alternatives for < 1s/day error:
If you prefer an SPI interface: DS3234
If you prefer a smaller package (8-pin instead of 20-pin): DS3231MZ

Thanks guys! I'll go for the DS3231 then.

Would this module be OK? http://www.ebay.com/itm/171366856553

Zealot:
Thanks guys! I’ll go for the DS3231 then.

Would this module be OK?
http://www.ebay.com/itm/171366856553

That module would be fine and it also has some non volatile memory you could store data in if needed.