What's the best form of cardio or metcon for a lifter? I'd say the resistance bike, like an Assault AirBike or the old-school Airdyne.

Why? Well, let's assume we're talking mainly about having a positive impact on body composition while also getting in better shape. What are we looking for that'll accomplish this? Four things:

1. High Intensity: You want something that delivers maximum results in minimal time. An exercise requiring more than 15 minutes of work can have a negative impact on muscle gains, especially if done in the same workout. So if you can't go long, you need to be able to go hard.
2. Sustained Metabolic Stress: You need to be able to sustain that high level of intensity long enough to cause metabolic stress and force adaptation. So we're talking 60-90 seconds of very intense work, or up to 3 minutes at a high level of intensity. This should be the duration of a bout of conditioning work (you can do several bouts in a workout) for maximal impact on body comp.
3. Whole Body Involvement: You want maximal efficiency: the more you can accomplish with the least amount of work, the more of a positive impact you'll have on body comp, meaning that you have less of a chance of losing muscle or halting your gains. Involving the whole body also produces a much greater hormonal response and leads to a greater energy expenditure per unit of time.
4. Resistance: While unloaded conditioning work can be effective for fat loss and get you in shape, it doesn't do anything for muscle mass. Resisted metcon exercises will at the very least contribute to muscle maintenance and can even help you gain some muscle.

With a resistance bike, you can go really hard. The harder you go the more resistance you have. It also involves the whole body (these types of bikes have moving handles), and it can be done for the required time periods.

The rowing ergometer would be a close second, but only because the resistance felt is smaller.