Which one is better to say? Volt or Volts?
(When I gave this answer, I had in mind that, with your "Which is better to say?" you were meaning when writing. Now I see in your comment to Tony that you indeed mean when talking. When talking, W5VO's answer, in my opinion, is correct. My answer is still useful if someone is interested to know how to write them.)

Regarding case, unless that word starts a sentence (which is not the usual case), it should be written in lowercase. See Tables 1 and 3 in this page from the National Institute of Standards and Technology (NIST). So, when referring to the units themselves (without any quantity implied), you should write "volt", "volts", "joule", "joules", "watt", "watts", "ohm", "ohms", etc. All in lowercase.

Regarding singular/plural, see point #5 in this other page (also from NIST) to see an example ("The speed of sound is about 344 m·s-1  (meters per second)") that shows that plurals should be used for unit names, when needed.

Regarding how to express values of quantities, point #14 says that you should always use Arabic numbers and unit symbols (not unit names!).



#14 : Numerals &amp; unit symbols
Values of quantities are expressed in acceptable units using Arabic numerals and symbols for units.

proper:
m = 5 kg
the current was 15 A

improper:
m = five kilograms
m = five kg
the current was 15 amperes (incorrect!!!)



So, to answer your question, you should write "2 V", "1 V" and "0.5 V".

"2 volts" / "2 volt" / "2 Volts" / "2 Volt" would be incorrect.