The way I see it, there are a few possible answers, and I'm curious to see, just on a personal preference level, which ones people lean toward.
Now the obvious one is Takanuva. Toa of Light, yadda yadda yadda. Now, since he was intended by Artakha and/or the Great Beings as a failsafe in the same way as the Toa Mata were, the fact that he is transformed from a Matoran like all the more average Toa out there does kinda diminish his impact, especially since it was implied it didn't have to be Takua specifically; any Av-Matoran would have done. Still, it being Takua does give him a more personal connection than most of the others.
The next most viable contender is of course Helryx. She's the only one aside from the Toa Mata to have been built directly instead of transformed, and while distant from the team (and arguably redundant since she and Gali share the same element) due to her duties as leader of the Order Of Mata-Nui, she still has that inherent connection to them that even Takanuva lacks.
For a looser case, there's Toa Ignika. While not technically a Toa so much as a Toa-like body constructed by the Mask Of Life, it still got inducted as an honorary member shortly before its sacrifice. While brief enough that its presence on the team doesn't necessarily clash with the others' claims, this is operating under the assumption that there's no slot for Eighth, Ninth, etc. members.
And finally, laugh if you will but I'm being completely serious here, there's Voriki. While her claim to the spot is rather hampered by the fact she doesn't technically exist, she still managed to get the spot in Brickonicle, and does have at least some degree of potential for the role. If nothing else, she's the first claimant, with the hoax being widely believed possibly before Takanuva was even a gleam in Lego's eye, and in a roundabout way seems the most fitting with the tribal theme of the early years.
Also, there's Ekimu, who basically filled Takanuva's role as G2's Toa of Light, even if he denied being one.
As an honorary mention, there's also Ahkmou, who was rumored to be planned as a Shadow Toa antagonist to Takanuva had G1 not been cancelled in '09. While his villainous role naturally precludes him from being counted as a member of the team, and his rivalry with Takanuva also means that the seventh spot is already taken, he still would have had a unique role that, much like these other examples, puts him a step or two above the more rank-and-file 'average' Toa that lived elsewhere in the GSR.
So what do you think? Who is the seventh Toa? Who would you like to see be the seventh Toa? Is there anyone else I'm forgetting who could arguably lay claim to the spot? Would saying 'all of the above' and giving the Toa Nuva ten or so members be really stupid or really awesome?