It depends how you define "top e-liquid"
If you mean best possible flavour then you'll want one of the little 'artisan' liquid makers that focuses solely on small batches quality of ingredients that kind of thing.
If you mean "least likely to make me ill" and want e-liquid that's produced in clean room conditions, is very regularly tested to high standards with GC-MS machines for impurities and other nasties, then you're looking at WizMix, Decadent Vapours, Alchemists Cupboard, T-Juice Dekang or Hangsen (others might do that too but that's all I can remember off the top of my head) basically as clean rooms are *expensive* and a GC-MS test will run you £200ish per batch tested which puts that far out of the reach of the small artisan makers that focus on small quantities
I think there is nothing wrong with e-liquid produced in what amounts to a very clean kitchen, using good quality ingredients. We all smoked cigarettes and inhaled much worse compounds for god knows how many years. Your mileage may vary.