Which is more anonymous? About bitcoin,
Which is more anonymous?

1.using mixing service(I heard there are many fraud)

2.buying with amazon gift card in Paxful
In some cases. Mixing services have actually been shown to be able to trace the inputs all the way through the many outputs until its final destinations.
Its hard work for a person, but there is software available that can do it with good success rate on several mixers

Buying some giftcard is closer to the more obscured route of transfers, where you simply change your currency into another, and use this until you need to change it back some other place. A gift card would probably give to big a loss. A far better improvement to this technique, would be to use one of the few cryptocurrencies that offer anonymity, like Monero or Dash. Then you could use the route:

BTC (1) -> Monero (1) ---anon-send---> Monero (2) --> BTC (2)

Of course if you are able to use Monero or Dash instead of Bitcoin, you can simply just keep these and skip the last conversion back to BTC. This will make it cheaper when using it as payment, since you don't get stuck with the extreme-fees situation that Bitcoin is facing right now.