Dont want to dumb down the question but when you say "best", best for what?
Bashing,club racing, nationals?
Or best from which company?
If you are club racing it is crazy to be spending your hard earned on top spec matched cells. Chances are you are going to notice little,or no difference between a good set of matched cells and soldering up some unmatched loose cells if you're at club level.
Yes, IB42's can be unstable but with good soldering technique (good solder,bars and iron,scuffing the cell ends and not heating the cells up for too long) get yourself a good few loose cells for 2 sets of 6 cells and see how you go.
If you are racing with a demanding 3.5 or similar though, then yes my opinion would change greatly. A 10.5 should be fine with unmatched cells at club level, you will still have power left at the end of the run.
I guess it just depends how much more money you want to throw in for matched cells to get the extra "zing"