Chandra Ablaze Chandra, Flame's Catalyst Chandra, Flamecaller Chandra, Heart of Fire Chandra, Spark Hunter Daretti, Scrap Savant Jace Reawakened Jace, Cunning Castaway Jaya Ballard Kaito Shizuki Kaito, Cunning Infiltrator Kasmina, Enigmatic Mentor Lord Windgrace Lukka, Wayward Bonder Nahiri, the Harbinger Nahiri, the Unforgiving Narset of the Ancient Way Ob Nixilis, the Adversary Oko, the Ringleader Professor Onyx Quintorius, History Chaser Ral Zarek, Guest Lecturer Ral, Crackling Wit Sarkhan, Fireblood Sarkhan, the Dragonspeaker Tamiyo, Collector of Tales Teferi Akosa of Zhalfir Teferi, Master of Time Tezzeret, Betrayer of Flesh The Royal Scions Tibalt, the Fiend-Blooded Urza, Planeswalker
There are a few Otters and related cards that really stand out, though
Deaths Shadow was THE defining card for the tempo archetype in Modern and was a legit tier 1 shell for years
Archon of Absolution Archon of Absolution was a solid sideboard card against white decks when it was legal in Standard, and solid in the early Historic metagame from time to time
