If I wanted to say partner in Spanish, referring to someone who I'm in a loving relationship with (oppose to a business relationship) would I say pareja or socio or compañera? If they are all of equal meaning, which would imply best that I'm talking about my spouse/girlfriend?
This post is part answer, part long comment on Pablo's ... Notice that he's from Argentina and I'm from Spain, we share a language but our actual cultural background differs.
In Spain, based on my experience ...
socio/socia is a slang term if you use it to designate your spouse/girlfriend. It's not commonly used.
pareja is the most common used and it's the best one in my opinion, I agree with Gustavson there. It means that you got a romantic + sexual relationship with someone. Without further context and no modifier, it does not implies just that you maintain sexual intercourse regularly with someone or that you share a space for living, it implies deeper ties. The exact implications and the deepness of the feelings is not implicit in the term. You have to ask for such information to the specific couple and probably they won't answer.
compañero/compañera got some liberal or leftish connotations. I think that the term begun to be used in this sense during our Second Republic (1931-1939) meaning that the relation was exactly the same relation that may have a married couple sanctified by the Church but without such sanctification. It was used to remark that they didn't need sanctification at all from the Church and from no one else but themselves. No rightish person is going to use that term here in Spain. Probably they would use novio/novia, esposo/esposa, marido/mujer depending on its status.
Those terms novio/novia, esposo/esposa, marido/mujer are not used exclusively by rightish people. Leftish, rightish and apolitics use them. Being marido/mujer the most used if they are married.
novia is the usual translation for the word girlfriend and its use is very common too.
As an example, let me quote José Mújica, old guerrillero, admired political leader and ex-President of Uruguay, another Spanish speaking country:
¿Cómo conoció a su esposa, la senadora Lucía Topolansky?
-¡Disparando! ¡Disparando! Andábamos disparando por el monte. (Sonríe) Lo que supera la realidad de lo que pueda pensar cualquier novelista es que Lucía fuera la encargada de ponerme la banda presidencial. Cuando fui senador me tocó investir al primer presidente de izquierdas del Uruguay y después, mi compañera Lucía, al ser la senadora más votada, tuvo que investirme a mí. Ahora empezamos a estar ya un poco pasaditos de años...
Socio or socia is really for business partners, and although it can also be a slang term for a spouse or romantic partner, it's not universally understood and may sound too familiar.
Compañero, compañera are much more commonly used for this meaning, although they have exactly the same problem as English "partner" (but context usually resolves the ambiguity).
Pareja is basically unmistakable; it can mean "dance partner" but of course that's a very restricted context, so most everybody will understand that it means romantic/sexual partner. It does have (at least for me) a certain ring; semantically, it points specifically to coupling (in every sense of the word).
Compañero/a on the other hand signals companionship, togetherness, and sharing, so to me it sounds better. Pareja can mean "someone I meet regularly for sex" or "someone I share a place with"; compañero/a invokes deeper ties.