Positronics is one of the oldest seedbanks in Holland, if not the oldest. They once had one of the most extensive seed catalogs available, but most was lost in a fire back in the 90s, or so.
I've never seen any of their stuff ran personally, but they have a good rep. and I would recommend to try them, before some new company you haven't heard of. That is, if you are just looking to try something new. Otherwise you could go with someone that is almost always a winner like: TGA, DNA, Dinafem, reserva, or sannies. to name a few. Any of those guys and 90%+ of the time you will have great results.