[FabHK]

Unless sub-second accuracy is required, I'd be in favour of UT1. It doesn't use SI seconds (while TAI and UTC do), but it has the advantage that a day has exactly 86400 seconds, so you don't ever have to deal with "23:59:60" timestamps.

Summary:

UT1 - noon is when the sun is above you, day has 86400 seconds (not SI seconds)

TAI - noon drifts away from when the sun is above you, day has 86400 SI seconds

UTC - noon is when the sun is above you (+/- 1 second), day has 86400+/-1 SI seconds

Difference UT1-TAI: continuously diverging

Difference UT1-UTC: continuously diverging up to less than +/-1 second, then discretely jumps back when it gets too big (leap seconds)

Difference TAI-UTC: increasing in discrete increments of +/-1 full second (leap seconds)

[Unklejoe]

I'd rather just use TAI and never have to worry about anything. Let the conversion to UTC be the client's problem.

Having the length of a second always be the same is kind of important.

[FabHK]

For logs, fine. But you do deviate more and more from what the world is using (as of 2017, TAI is exactly 37 seconds ahead of UTC, according to Wikipedia)