Realise that the lowest price of a stock at post market hours is different between different brokers. Does this only happen for pre and post market or? Which broker is then better in getting low px?

Pre/after market price is determine by ECN network, and there are a few ECN provider.

All the price is actually very similar for diff ECN, just that ECN network can easily be jam, thus even tho some brokers using same ECN might also shows diff price, but actually the bid/ask behind it still the same,

broker dont decide the price, most of them are using NYSE Arca

After-hour stock prices are quoted by brokers because the stock market is already closed. Therefore the prices can be different from brokers to brokers, it depends on demand and supply forces in that broker. Don't think you can identify any broker that always quotes better prices.