I am a working professional who has  BSc in Graphic Design and a MA in
Marketing and Communications Studies.  In five years, I will be moving
with my wife (who's English) to the UK. I would like to make myself
viable for the UK job market and feel that learning another European
language would benefit me greatly both professionally and personally.
However, since I have not lived or worked in the Uk, and the most
language experience  I have is two years of  Latin, I am largely
clueless about which language to pursue.  Given what you know about
me, which language would best improve my odds of finding employment in
the UK?

I don't think that the lack of a second language will be any barrier
in the UK now that English is the Lingua Franca of the world (if
you'll forgive the use of a foreign phrase).

At one time, French was a must but no more!

Maybe when you get a job, something will suggest itself but I would
certainly not study any language on the off chance that it might be
useful.

Bryan
(English born & bred and UK based but with some Latin, French,
Spanish, German and Dutch)

While not necessary by any means, I would recommend Spanish or German
on the grounds that they are widely spoken and relatively easy to
learn. Spanish would be ultimately more useful as it is spoken across
continents while German is a bit more confined to central europe and
has grammar that will make you tear your hair out :).
For a native english speaker there isnt much point going for a more
exotic language like Swedish, Dutch or Finnish etc., as they are
difficult to learn, pronounce, write and the youth in those countries
have excellent english already.

Unless you are extremely linguistically talented, then I doubt that
you would be able to get sufficiently proficient in another language
for it to have the slightest effect on your job prospects.

For personal satisfaction and to make holidays a little easier you
could learn Italian, Spanish or French. Which one depends on your
holiday preferences.

Possibly, if you intend returning to the USA, Spanish might be the most useful.

Hi, I'm an American living in Germany for decades.  
First off, in your field, professionally you will probably find
English quite adequate on the Continent.
I suggest Spanish  - for a couple of reasons:

The Germans are pleased when a foreigner speaks German, but all of
them have had to learn English in school, and those you will probably
interact with will be more fluent in English than you will be in
German and will be pleased to speak English.

The French are sometimes a bit snooty about their language and less
appreciative of foreigners's efforts.  Unless your pronunciation is
correct, you may be willfully misunderstood.

Spanish or Italian may be more useful, but as Frde suggests, more for
vacationing, whereby I agree that Spanish will have greater use in S.
America and maybe in the States  - and you will probably find a better
selection of places/ways to learn it where you live, and to practice
it.

Do make the effort.  Without a second language, one feels the butt of
all those comments about lazy English speakers.  With a foreign
language  - even the wrong one at the moment -  you will feel more
comfortable and free of such accusations, and you will become
acquainted with the culture of that country, which will probably
direct your interest and travels.  (So, if you know that you prefer
Italy, choose Italian.)

Hmm? - I am surprised that my friend Bryan didn't suggest that you
learn British English first ...  ;-)

Cheers, Myoarin