1) The Ski Chalet right off of I-270 on Muddy Branch Rd in Gaithersburg; and,
2) Princeton Ski and Sports on Little Patuxent Pkwy in Columbia (right off of MD 29, just below the intersection with I-70).
Both are easy to access when you are traveling I-70/270 up to the local ski hills.
With respect to your question about MLK weekend crowds, yup, this will likely be one of their busiest days of the year. Its hard to predict lift lines, but you can probably expect 10-15 min waits during the busiest parts of the weekend. If I had to pick between Liberty, WhiteTail and Roundtop, I'd definitely go with WT.
Tom / PM