Maryland has a lot of good tree companies, unfortunately on the eastern shore there are more butchers than arborists. Between Baltimore and DC there are a number of reputable companies. Bartlett has a few offices. Carroll Tree and A and A are pretty good. Richards, Ballards, and Arborquest serve the Annapolis area and they do pretty good work. Thats just to name a few that I have come across. I believe Davey is doing alot of work on the eastern shore now and they also have an office out of western baltimore county.
It really depends on what you are trying to get into and what you are trying to avoid. If you are looking for IPM, going with someone large like Bartlett or Davey is the absolute way to go in order to get your feet wet. They also tend to offer more in the way of young tree pruning. You also need to decide how important "safety is." Very few companies enforce mandatory hard hats, chaps, safety eye wear that is required by ANSI. The big boys and the really "forward" companies tend to enforce those rules, whether its because of liability or the reputation of safety, is for you to decide.
People will bash the larger tree companies for any number of reasons, but at the end of the day, you're opportunity for advancement is obviously much greater with them then with a small company. Anyone who says this isn't true is basically uninformed as there is only so much room for growth when a single office is involved and someone owns the company and is also doing the work and not focused on the actual business of selling work, marketing, billing, etc.
You really need to decide if you are an arborist or a tree guy, after you make that decision you can start to look at different tree companies and select one that is a good fit for you.
Here are a few things you may want to ask of your potential employer
Do you offer a full range of tree care? Removals, pruning, IPM, fertilization, air spading, stump grinding
What type of equipment are you using? Trucks (bucket vs chip body) Wood handling equipment, stump grinders, air compressors, spider lifts, sprayers
What type of safety and PPE standards do you practice?
What opportunities do they offer for advancement?
What is their drug and alcohol policy?
Does the company offer benefits? Health, dental, vision, 401K