Honestly, it depends on the distance you are looking at. I am lucky enough to live in Howard County so I am a big fan of the CTA Triathlons, mainly the Columbia Triathlon. However, we went last year to Deep Creek and competed in Savageman as well, which has a really cool course, and can be a great event if you want to bring friends and rent a house out there.
What kind of distances are you looking for? I'll give you a few other options if you would like to hear them .