If you do your ear mold impression right (by going through a audiologist), than the chance of getting your custom fits right in the first trial will be high. If you DIY'ed your own ear mold (which some companies offered for lowering cost), there is a higher chance there thing might goes wrong and you will have to send in your custom for re-fitting. I won't recommend the DIY method.
'Best' usually involves personal taste and it is hard for people to admit that the custom they paid a few hundreds to thousands of dollars for might not be the best there is. Of course, since you can't really audition a custom without actually getting the custom first, most of the time it is just a leap of faith on reviews to determine which IEM to buy. If you don't keep a cash cow in your backyard, than I'll suggest you start by looking those under $400 deals before investing in tons of money.