So I want a VPS for proxy purpose, I don't care about memory/cpu/storage at all, what I want is a vps provides highest bandwidth/traffic for the lowest price.

From what I check:

Hetzner: 20TB for 3$

OVH:  100 Mbps/Unlimited for 6$

Netcup: 40TB for 3$ (too complicated to register)

Ionos: 400Mb/Unlimited for 2$ (hasn't checked)

Are there any better options?
Honestly, I don‘t think you will find much offers beyond 20 TB/month besides the 40TB of Netcup. Even those providers with „Unlimited“ usually have some limits (usually hidden as „Fair Use“ clauses in their terms of service). Or, for example are too slow to actually even reach that much traffic

100 Mbps 24/7, for ex, would be about 33 TB in theory, but that would require 100% load all the time and for most applications persistent high load is unrealistic. With web-traffic you will likely see short spikes and more than 10% average utilization of that port isn‘t reasonable (even 10% are unreasonable if you consider user fluctuations through the day). And even at 1 Gbps @ 10% utilization, you will thus only use up a maximum of ~33 TB

The only way I‘ve ever seen VPS users goover ~10 TB is through file sharing (like with Torrent), as that‘s an easy way to put persistent and high load on any network port. (But if that‘s your goal you better explicitly state that, as their are many providers that kick customers for torrenting and that would further limit your list.)