I would like to know which model of Mikrotik routers would be best for my system. I have about 400 units in service and need something better than what I have now which is rapidly becoming less dependable. I have fiber connection and have 400megs at my disposal.

I have an old car and I need a new one. Which would be the best for me? Road in front of my house is 15m wide.

Well ask a stupid question,.....that is why I never like asking anybody anything on here.

That is how it is. If you do not describe your situation, none could seriously suggest to you anything. Look around on the forum how others describe their networks when they ask the same question like you do.
And I believe, if you read those posts, you will find the answer to your question, because you are not asking anything that was not already replied here.