Buy European

As each of them more or less specializes in a category, that's hard to say. Pantasy focuses on general models, with very special parts to actually convey the license, funwhole makes very neat house models, some with lighting (not two small LEDs, but dozens of LEDs with intensity and color perfectly matching their purpose and scene, with hidden cable management through special bricks) and other models, cobi makes very accurate vehicle models (including ships etc.) (mostly in system bricks), CaDa makes good vehicles too, but more in technic bricks (including remote control and actual technic functions). Mould King is more of a jack-of-all-trades, but overall cheaper (in my experience). BlueBrixx is more complicated to describe, as they have multiple series, including reselling other manufacturers (eg. Mould King), but they also have their own sets, for which they buy existing bricks (mostly GoBricks).

So overall: It depends on the type of set, but especially for large (also expensive, but FUCKING large) sets, BlueBrixx is best. Eg. Burg Blaustein, 26k pieces, 940€, and the astronomer's series, 21.5k pieces, 240€ (the next set for +>5k pieces and probably +~200€ was already announced). But there are more manageable sets, like the V100/200 locomotives (~5.5k pieces, 200€ and ~8k pieces respectively). And of course, many smaller and neater sets.