Ich möchte dir eine Frage stellen.

I think it's : "darf ich Sie was fragen?"

Or: 'Darf ich dir/Ihnen eine Frage stellen?' . . .

"ich möchte dir eine Frage stellen" could easily be expressed in English as "I'd like to pose you a question". "to pose a question" is a fairly common phrase, but it sounds slightly formal. The more common phrase is "to ask a question". If you're talking to a friend or your family you'd say "ask", never "pose".

Thanks for the responses.

Eric, "eine Frage fragen" klingt im Deutschen einfach unschön, ähnlich wie "ein Essen essen" oder "mit/auf einem Rad radeln" oder "einen Ausruf ausrufen".
Es fällt mir schwer, mir einen Muttersprachler vorzustellen, der solch eine Formulierung verwendet, sei es in Rede oder Schrift.

Im Englischen gibt es dieses Problem dieer, ungemein redundant wirkenden, Wort(stamm)wiederholung zumindest bei "to ask a question" nicht.

"Ich würde Dich/Sie gerne mal was fragen" would be a common, slightly informal way to put it.

The others are correct too of course, but each has a slightly different tone to it:

"Ich möchte dir eine Frage stellen" sounds a bit like you are making an announcement.

"Darf ich Sie was fragen?" or "Darf ich dir/Ihnen eine Frage stellen?" would be something you might say if are going to ask something indiscreet.

But I guess that's the point of announcing a question at all instead of just plain asking it...

To ask a question = eine Frage stellen
to pose a question = eine Frage stellen
In German you always say: eine Frage stellen. That's not formal as there is no other possibility. If you talk to friends or the family may be you'd chose other phrases at all. In common speech/slang there are lots of possibilities.

Very informally, I think one might say: "Kann ich Dich etwas fragen?"

Or just: "Sag mal..." if it's something fairly trivial.
But I'm not a native speaker of German, so I'd be glad to be corrected.

@ EMC: Those are fine and very common (though informal).