I am about to start a new game project and I'm seeking your input on a design aspect.

The game includes a base-building section with the following options:

1. The base is a starship: building takes place inside the starship.
2. The base is a floating island: building occurs within the floating island.
3. The base is a sea ship: construction is inside the ship.
4. The base is an asteroid: building inside the asteroid.

Which option appeals to you the most at first glance? I'd love to hear your suggestions.

By "inside building," I mean similar to the construction method shown in the image.

https://vinaygao.itch.io/

The ones that appeal to me are floating island and asteroid. These two gives impression of uncharted worlds so building base (or anything) there would be a challenge and adventure. Not to mention, both locations could have resources which could be used for building or at least be traded.

You mean like the xcom bunker thing?

That would depend a whole lot on the game and setting. It seems you do not even know if your game should take place on the ocean or deep space. Or if your "base" is a moving thing or a kinda fixed base.

A derelict alien space station.

Something like Babylon 5, only smaller. The rotation gives you free gravity and an excuse to flatten the 3d aspect to the surface of the cylinder. That is what bothers me about such layouts. Bunkers, buildings, ships, asteroids and so on are not 2d.  But it quickly gets confusing if you really give players three dimensions. As a game mechanic 2d works of course, even with people knowing bunkers are 3d.

I would dislike the star and sea ship for this reason: whatever plot you have, there might be the issue of explaining, why you do not just move the ship there. On the other hand, maybe your plot does revolve around moving the big ship (or asteroid)  somewhere and your missions involve a lot of fuel gathering.