Ooh, that's an interesting question. I'd have to go with highest total energy consumed through their metabolism and activities. Those with the highest chemical impact on the planet.

I would go with the highest biomass :-). Whoever weights most in total wins.

EDIT: According to https://en.wikipedia.org/wiki/Biomass_(ecology)#Ocean_biomas... it's earthworms.

Is a giant sequoia tree, the largest of which was able to accumulate ~1350 tons over 2000 years, more successful than the quantity of worms that lived under it during that time? A continual measurement per unit time seems more fair to me than instant current weight at any given time. Maybe if it was possible to measure the biomass generated over the course of the year for each species. I'd think that's directly related to their energy consumption.

Why is that even the right direction of the axis? Couldn't you argue that the less energy you need for all your species' activities the better, if all else is equal?

Because energy is a limited resource, ability to extract a higher share of it implies competitive success. Also, it's possible to convert and directly compare the energy potential lost to the environment through the various mechanisms for metabolism and activity.

Please don’t backward engineer the metric so that we will win ;-)

Yeah, factoring in activities certainly gives us the advantage. I suspect most people have an idea of success more along the lines of mass of other species consumed. That seems unfair to plants though.

I feel some measure of chemical impact of the species to be the most level playing field, and that relative total energy consumption is close to the core of the question.

I would go with the time fitness takes to bounce back after an environmental change...

So us, then bacteria, and so on.