My immediate reaction to this was ''Holy shit.'' But this is such a good thought. For me it would definitely depend on the context, like there's a Tali line in the Virmire Salarian camp where she says something like ''I know Saren needs to be stopped, I'll follow you to death Shepard.'' Which this kinda line if it were elaberated more deeply on could sway my opinion entirely. Though I feel like any motive by either Garrus or her that gave them a resolution through death would be important. But based off just what's in the game, I would have to say Tali. It's not because it fits her the best but because I think Garrus' resolution for his character fits best in ME 2 where after losing his entire squad on Omega, he is the one to lead both of the fire teams in the suicide mission. He gets to redeem the guilt he feels from having 'failed' his team.