It depends what you are looking for, lots of price differences...
1. Belle Verde- right by Greenspring Station and Falls Rd.
Outdoor ring, no indoor, some trails, large fields, pretty nice.
2. Rockland Barn - self care barn on Falls Road in Towson. Old stone barn (I think it's stone), no trails, no indoor or outdoor, grass ring, I believe. Met some nice people that board there. I think between $250-300/month
3. Thornridge Manor- upscale dressage barn, outdoor and indoor with fabulous footing. Everything you could want. $800/month
4. Do NOT go to Graham Equestrian Center...PM me for more info. if you want to. Yes, I know from because I was a previous boarder in 2008. I still know people that board there, and they are ONLY there because it is right by their house. I still wouldn't do it for that reason...many safety issues.
I know many places in the area...the Equiery has several places listed that do not exist anymore or have changed names. Let me know if you need any information. Monkton (20 min. from Towson) has a bunch of farms that board, but it really depends on what you want.