What is your most accurate mechanical watch?

All three (911, 1861, and the 1045 Grail in my avatar) are about +10 sec/day.

My Seamaster Professional with the 1120 was keeping time within a second or two a day until it wasn't. We'll see how it is when I get it back from servicing. My Railmaster LE is about +3 on the wrist. The others aren't quite so close but less than +5, usually.

I seem to be pretty consistent in my habits so my watches are fairly stable.

My Co-Axial Speedmaster gains about 1 minute a month when I wear it daily. Works for me!

Given a full wind once per day this one gains a consistent 1.5 seconds per day. Not bad atall for an over 50 year old.

For me it's a Steinhart Ocean One Vintage Red, the ETA movement loses less than 15 seconds per month.

I have a old Soviet Pobeda I got in NOS condition years ago. In a 28 day test it stayed at a steady -2 seconds every seven day week, 8 seconds during the entire 28 day test. That was leaving it dial up when not winding daily or checking the time.
Not quite that accurate if worn all day but still far more accurate than one would expect from such and old and simple movement.
It is extraordinarily clean on the inside, so I figure it was serviced not long before I got it. It came in a factory delivery box with no sign of ever having been on a band of any kind.

I also have a refurbed Fortis that proved uncommonly accurate. It would gain 1 second every five days.
Unfortunately it began to lose about 2 seconds per day after I switched out the bracelet that was on it for an adjustable expansion band.
No idea why it would do that unless there was some tiny magnetic charge in one band or the other. I've demagnetized the watch with the new band with no results.
The band is very comfortable so I wear this watch quite often. Being a refurb I'm less bothered by the possibility of it being damaged.

My Slava seems to be staying within +2 seconds per week if worn about 12 hrs a day and left dial up overnight. There seems to be very little positional error. I had not worn it much after I first got it because the bracelet it was on was not comfortable. With a new leather band it seemed to become more accurate the longer I kept it wound and running.

Around 2 secs per day

For me it's my Tudor BB MT5602. Crown down, crown up, on the wrist... Whatever position, it runs at 0.2spd.

It’s a three way tie between my Rolex 1675, Omega Speedmaster 105.012-66 and my Seiko diver 6105-8110. All wearing on wrist.
Either one in the course of a month loses between 15 and 30 seconds. The Omega just had serviced in June, the other two, just keep on ticking.

Whereas my AP, which I’m currently wearing, loses about one minute per day... time for a service. Who knew?

I'm going to measure and track my watches to find out semi-scientifically, but I have a suspicion it's going to be a cheapo Flieger with a ridiculously well-regulated Miyota 9015 (although I'm rooting for my Speedy, which is now ticking at +2.5spd but needs more data points).