Zigzag
Up, down, up, down — never a straight line.
The comparisons between neighbors must strictly alternate: every third digit forces a turn (a<b>c or a>b<c, all the way), like 1324, 3142 or 102030. RNGdle requires at least 4 digits.
Your record
Zigzag is the opposite of "locally monotone": neighbors must never tie, and the direction must reverse at every step. There are only two opening stances — dip-then-rise (1<3>2<4) or rise-then-dip (3>1<4>2) — and whichever you pick, you dance it to the end of the string.
It's the friendliest feature in the sequence category: 136,425 across the range, about 13.6%. Shorter is easier — 3,105 among four-digit numbers versus 114,381 among six-digit ones, since every extra digit is one more mandatory turn.
Zigzag and Alternating Parity often show up together but imply neither: 696969 hits both, while 1324 is a zigzag with no parity alternation. And 101010 is a triple crown — Zigzag, Alternating Parity and Periodic at once.