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