Swimming at the Ten Ton Gym. Note that both that and Tanis (and pretty much every single other stat increasing effect in the game except leveling up) use Slow Stat Gain, which gives diminishing returns the closer you are to the stat cap, and that Tanis will only train you up to half your max Aim or 25, whichever is lower (which, AFAIK, is the only Slow Stat Gain in the game capped below 100% of max).