There is no fixed criteria for selection of firm for articleship. I would suggest you to go for a firm which provides you good exposure in tax and finance.
You may try for BIG 4 firms. But do not leave the chance if you are getting the opportunity to work with a good but medium firm. However there is not much benefit to work with a firm or CA where there is not much work.
You may take transfer during first year of articleship as the same is allowed with ICAI permission.