What "alternate" form of life do you think is most plausible?

[JMBuilder]

Just a random science theory discussion. What different kinds of life could there be in the universe or possibly even on (or inside; I'll get to that) the Earth? What do you think is most scientifically plausible? I'll go over a few ideas for discussion.

1. "Cold" carbon-based life
     Probably the most plausible theory, describing something similar to our biochemistry that thrives in lower temperatures and that uses a solvent other than water, such as ammonia. Saturn's moon Titan seems to be an interesting candidate for theoretical life like this with its freezing mess of hydrocarbons and nitrogen compounds.

2. Silicon-based life
     This is the most commonly proposed alternative biochemistry since silicon shares some similarities with carbon. While they do form the same number of bonds, silicon is not nearly as chemically versatile as carbon. However, interesting bonds can be formed with alternating silicon-oxygen atoms, quartz being a good example of this. In a very hot environment rich in hydrogen sulfide or other sulfur-based acids, these chain silicates might be able to form more complex structures. There are volcanic regions on Earth that seem to match this environmental description. Is it possible that there is a subterranean biosphere of silicon-based life on Earth?

3. Boron-based life
     I find this to be one of the more plausible theories on non-carbon-based life. Boron forms very interesting bonds, namely the three-center two-electron bond, making it quite versatile. Boron compounds are more stable in very low temperatures, and interestingly enough, they have an affinity for liquid ammonia as a solvent.

4. Metal-based life
     Certain metals can form amazingly complex atomic structures that rival those of carbon. In a blistering hot environment where molten metals exist with some solid metals mixed in, complex metal structures might be able to form.

5. "Radiophilic" carbon-based life
     Based on fiction as well as a theory of my own. There are some radiosynthetic fungi that absorb nuclear radiation to produce nutrients the same way plants and other organisms absorb light or like how we use light to produce vitamin D. A carbon-based lifeform with a "radiophilic" biochemistry might be able to inhabit planets orbiting giant stars, thriving on their intense radiation.
     Being a childhood fan of StarFox games, I came up with a more feasible explanation for life in the Lylat System, "redesigning" it to be something other than a bunch of corny anthropomorphic animals. Since the main star of Lylat is a blue giant, lifeforms in the system are radiophilic and carbon-based with six-nucleotide DNA, giving them incredible diversity in appearance.
     A more well-known example of a theoretical radiophilic lifeform is Godzilla; basically a walking nuclear reactor that can vomit radioactive destruction.

Any more ideas? What might be able to form life as we don't know it?

[Austritistanian]

1

[Dunkleosteus]

I would say both one and two are fairly plausible.

[JMBuilder]

I've got another idea. I've edited the original post.