With the following 'asterisk': The first half of the original series. Up until then, the manga and the anime were on the same page. When the anime caught up, the writers made their own ending. I personally like it, some people don't, but it's not the 'true' FMA ending. Many interesting and relevant things happen to the brothers in the first few episodes of Brotherhood that seem hurried because, at least somewhat, we'd seen these adventures before.
But not all of the same characters, locations, and plot threads carry over between the two series. So just make sure to catch at least the first half of the original anime (up til around "All Is One, One Is All" or hereabouts) after you finish Brotherhood! ^_^
Personally, I recommend watching FMA: Brotherhood instead of FMA.
I disagree completely, IMO the first FMA was far superior in terms of dramatic pacing. FMA:Brotherhood had a tendency to rush through parts that originally had a lot of feeling and sometimes inappropriately broke the tension by drawing out a gag too long.
They both definitely have something to offer, though, I'd just go with the first FMA anime first...
