APBRmetrics

What is the best signal a team will win a title next yr, year after or either? What has highest correlation:

last playoffs advancement
adjusted regular season srs
adjusted playoff srs
RS or playoff off or def rtg
ts% differential
star player winshares or rpm
top 3 winshares or rpm
record against defending champ
RS wins
RS or playoff clutchness
record or adjusted srs against top 10 or 5 in general or top 3 within conference
past record of coach and / or gm
market history or size
q rating of superstar
whatever else you can think of

The best signal is that they won a title previously

Almost certainly, forgot to list that as I was focused on seeking first in current window (3,5,7 yrs).

if one had the data, what is the best signal of repeat?

Here's an impressive one. Teams who've had the best record in the league (or tied for it) at least twice within a 5 year timespan in the shot clock era:

57, 58, 59, 60, 61, 62, 63, 64, 65 Celtics: Titles for everyone!
66, 67, 68 76ers: Win 67 title
71, 74 Bucks: Win 71 title
72, 77 Lakers: Win 72 title
73, 75 Celtics: Win 74 and 76 titles, weirdly
75, 79 Bullets: Win 78 title
80, 81, 82, 84, 85, 86 Celtics: Win 81, 84, 86 titles (Best record 6 out of 7 years = impressive/underrated, by the way)
81, 83 76ers: Win 83 title
87, 88, 90 Lakers: Win 87 and 88 titles
92, 96, 97, 98 Bulls: Win 92, 96, 97, 98 titles
98, 99 Jazz: First Price is Right sad horn (they also tied for 1st both years for what it's worth)
99, 01, 03 Spurs: Win 99, 03, 05, 07 titles
03, 07 Mavs: Win the '11 title
09, 10 Cavs: The truest fail of the list with 2 outright best records in a row, no health problems, no lockout. But can still make up for it.
11, 12 Bulls: D-Rose screwed this one, but again, this ain't over
12, 14 Spurs: Win 14 title

So basically it's so hard to finish with the best record that if you do it twice, the odds of getting a title in that era have to be strong