I think the best approach is to first visit every dealer you can, and find out as much as you can about each brand. Establish your price range, then compare different manufacturer's in that range. There is no "best". You will find that a particular machine or brand "speaks" to you, and that is the one you should choose.
For me, it happens to be Bernina's. My 2nd choice would be Viking. But everyone has their own preference. Once you think you've narrowed it down, at least to a price range and brand you think you like, then start asking questions of owner's that have those models.