Full potential as of TPM -- that is, at the highest skill, focus, etc. to combine to great the greatest potential power that he could have, with the knowledge and abilities he currently has.
All things considered, though, it's rather irrelevant; Maul owned Qui-Gon, and it's been established that Qui-Gon would own Obi-Wan. That Maul did as badly as he did is probably because he underestimated Obi-Wan, and didn't put as much effort into the fight as he should have (and even then, he still won). It's obvious he didn't think very highly of Obi-Wan to begin with; after Maul knocked Obi-Wan down into the pit, he didn't even look to see if his enemy was dead or not. All those things show Maul's overconfidence and arrogance, and ultimately led to his downfall -- not Obi-Wan being 'better'.