As far as the suttas go, the most definitive thing I could find were several in the Theragāthā and Therāpadāna of Khuddaka Nikāya. Now these are both considered later devotional texts, but nonetheless are the accounts of many arahants who were alive with the Buddha. The phrase "best of Buddhas" (buddhaseṭṭhassa) appears quite a few times. We have this in Mahāmoggallāna's verses:
Now this notion could very easily be disputed and argued using DN 28 and a parallel SN 47.12, where we find Sariputta roaring his Lion's Roar that there is no other - past, present, and future - who's knowledge of awakening that is superior to the Buddha. The Buddha leaves the implication to stand relatively unopposed in SN 47.12 (although Sariputta goes on to qualify it, gaining the Buddha's approval), but at the end of DN 28 - a sutta that begins with the same claim - he makes the definitive statement that there have been others and there will be others who have equal knowledge of awakening as himself. This is confirmed also more directly in SN 56.24. It is worth noting that based on the Vinaya verse already mentioned, this equal "knowledge of awakening" does not imply equal ability to pass it on. (Edit: I see you mentioned this above)Thag 20.1 wrote: Calm and quiet,
the sage in his remote lodging,
the heir to the best of Buddhas,
is honored even by Brahmā.
This "best of Buddhas" (buddhaseṭṭhassa) is also found in a few places in the Vinaya. Not sure if this is convincing enough. It is a rare phrase as far as I've been able to find. I'm pretty sure the first time I read Gotama being the foremost was in a contemporary source without much to back it up. I appreciate you taking me to task on this.