Most common subwoofer input?

I just bought a new house and I plan the build a 5.1 system in my family room. When I talk to my builder today, he suggests me to build a in-wall stereo cable for my rear speakers and subwoofer, so there will be no cable running around my family room. I know that rear speakers use normal speaker cable, however subwoofer have so many different type of input, such as: RCA, speaker wire, phono input or Neutrik Speakon. Which type of input is most common use for sub?
Thx for the suggestion!!!

Just want to make sure, I just NEED ONE RCA input (not a pair) right?
(amp -> RCA cable -> into one side of wall input -> output from wall -> RCA cable -> sub)

But I am wondering, if I don't have a receiver, and I only
have a stereo amp, can I still use RCA for my subwoofer?

For an A/V receiver: yes, just the one RCA connection.
For a "stereo amp" you'd have to be more specific. For example, if it has a set of variable pre-outs you could run two RCAs, or coax cables, to the left/right imput of your sub. Or you could run a second set of speaker wire from the amp's speaker posts in parallel with the speakers, if your sub has high-level inputs. But seeing as you originally mentioned a 5.1 system, I wonder where you're going with all this. You might want to consult a custom installer, as these are fairly basic issues.
If you have a designated "sub out" on a receiver that's all you need, just 1 RCA. Using a 2 channel stereo receiver or amp you will need a left and a right coming from a line level output. I'm not sure what you are running for equipment, if it is older with no "line level out" or "main out" you may have to go with my first suggestion and use speaker cable directly to your sub's high level input, then jump over to your speakers.
Hope that helps...good luck
Before I bought a house, I rent a small room and use my stereo system to listen music ONLY. In my new house, I plan to use my system to listen music and watch movie (50/50). I heard that if I want to use sub to listen music, high-level input is better than RCA; RCA input is good for movie, is it true? should I build both RCA and speaker cable for different purpose?
Please advise.
thank you very much!!
No, I wouldn't build RCA and speaker cable for each purpose. Use one or the other. I doubt very much you would even hear a difference between the two connections, I wouldn't stress yourself out too much. To make it simple just run 1 RCA behind the wall or through the floor joists and you are done.