preparing to play Half-Life for the first time

So, I was going through steam games to see if there were any titles i wanted to buy during the summer sale, and came across Black Mesa. Clicked it, and saw it was a remake of the original half-life...apparently minus something called Xenu(Not sure if that's a big portion of the game) and was going to pick it up, because it looked amazing! Turns out I already picked it up in January...I hate when I go to buy games that I already own.

So, anyway...I decided that it's high time to play the Half-Life series....I have Half-Life and it's expansion pack, as well as orange box..and apparently Black Mesa....So, should I play the original Half-Life, or Black Mesa? I'm assuming the original because of the missing Xenu thing, right?