The Briamosite (referred to in the novelisation of *[TAS: The Slaver Weapon][1]*) have to be serious contenders.  The story describes one of their delegation as being approximately **2.5 metres tall, dwarfing even [Lt. Saru][2]**

> "This is Sarvus, Leader of all the Briamosite systems, final arbiter
> of multiple-world decisions." **Pliver introduced them to the elegantly
> appointed, two-and-one-half-meter-tall Briamosite** who had stifled his
> laughter at Uhura-kirk's comment. "And Vice-Leader Chellea," Pliver
> continued, indicating the tallest member of the naturally towering
> alien delegation.

Although the novel doesn't end with an *explicit* agreement on a treaty between Briamos and the Federation to join, that was the purpose of the visit, which ultimately ended with a big thumbs up from the Briamosite delegation.


  [1]: https://memory-beta.fandom.com/wiki/The_Slaver_Weapon
  [2]: https://memory-alpha.fandom.com/wiki/Saru