Choosing a poker room suitable for grinding is very similar to choosing a poker room for regular everyday play. There are however some differences. One of the most important is that the most important factor to consider when choosing a poker room for grinding is that there are a steady flow of players to the tables and that there always are games available at your chosen level. Ideally a good poker room should have tables that stay busy 24/7 all year long. That way you can sit down at one of these table and stay at it as long as you have the energy to keep grinding. This will allow you to build up a big stack during your grinding session and a big stack has several benefits. Most importantly the respect it gives since players are less willing to call if they know that you can make them make a decision for all their chips at any time at the same time as your large stack proofs that you are a skilled player. Or perhaps I should say that it gives the impression that you are a skilled player. It is possible that you just have a good day but most players are as vary of calling a hot (lucky) player as they are a skilled one.
The importance of always being able to find good tables at the poker room can not be stressed enough as you as a grinder often will play long sessions and don’t want to be forced to stop playing or change table due to lack of competition. This can of course also be solved by being a member of and grinding at, several poker rooms but this is seldom an ideal solution either. Especially since it will require you to have more money tied down in poker accounts.
The availability of games is usually more important than the skill of the opponents when grinding. When playing regular poker you will be looking for a site with a lot of weak players that you can win money from. This is of course very positive when grinding as well but if the poker site offering the weakest opponents only have enough action to last a few hours a day it is not really suitable for grinding. It can of course still be a good supplement to the grinding. You are in other words not looking for the weakest game you can find when you are grinding, you are looking for a poker room and ideally a table that always offer a lot of action and that contains players that are weaker than you but not necessarily the weakest players you can find. Such a table might not be the best for quick profits but they are perfect for slowly grinding away at your opponents stacks and building your bankroll in the process.
The first poker room that comes to mind when thinking about busy poker rooms that always feature a lot of action is pokerstars and to a lesser degree party poker. Both these sites are usually good choices but there are a lot of other sites that might be good choices as well. Exactly which one is the best depends on where you are located, when you want to play, how long sessions you want to play etc. The important part is to make sure that you find a poker room that offers a lot of action, preferable good action,during those hours that you like to play