If you only have sixty seconds
Three of the eight are built to fit inside a minute: Outrank (ten rounds, no timer), Updraft (one flight) and Blitz (a hard sixty-second clock). Outrank is the one to send to somebody who claims not to like games, because it has no reaction component and no way to lose early — everybody plays all ten rounds.
If you want to think rather than react
Starlock, Summit and Ricochet have no imposed time pressure at all. Starlock's clock only measures you; Ricochet has no clock; Summit's time penalty of two points a second is the weakest term in its scoring formula, which is why rushing it is usually the wrong instinct. All three reward sitting still and looking.
If you want the hardest one
By raw skill ceiling it is Perimeter, because holding a kill chain through a boss fight requires you to ignore the thing that is most obviously demanding your attention. By failure rate it is Updraft, where most first runs end at gate 8 — the point where the gates start moving vertically.
If you want to beat a specific person
Any of them. Every result screen builds a challenge link that carries the exact board, course or deal plus your score, so whoever opens it plays what you played. Nothing is uploaded — the challenge travels inside the link itself. The three with the least luck in them, and therefore the most convincing head-to-heads, are Starlock, Outrank and Dash.
The one thing all eight share
Every game is generated from the calendar date, so every player in the world gets the identical run on the same day. That is the reason a score is worth sending to anybody. If you would rather not wait for midnight, each game also has an unlimited mode that generates fresh runs on demand, free and without a daily cap.