What is the fastest way to transfer huge files beween two android powered devices? What is the fastest way to transfer huge files (e.g. 500MB) between two android powered devices? Bluetooth? WiFi direct? Beam? or maybe USB on-the-go?
There are many variables that affect the performance of each method you've mentioned.
Additionally there's also the necessary configuration and requirements that each device must have in order to have solutions like this working.
As requested, lets summarize and compare possible performances between USB; Wi-Fi and Bluetooth:

USB:
USB performance can be extremely low or very high, depending on the available USB version, which in turn, combined with the SDcard reading/writing speed on each device allows us to accurately measure and ascertain the transfer speed.
There's also the physical limitation of the USB cable length that for some scenarios becomes a limitation.

USB 1.0: 1.5 Mbit/s (Low-Bandwidth) and 12 Mbit/s (Full-Bandwidth)
USB 2.0: 480 Mbit/s (effective throughput up to 35 MB/s)
USB 3.0: 5 Gbit/s (625 MB/s) which is more than 10 times as fast as USB 2


Wi-Fi
Wi-Fi loses limitations like cable length, but then again we need to account for the 802.11 protocol being used, the signal strength and range to accurately measure results:
┌────────────┬─────────────────────────────────┬──────────────────────────┐
│  Protocol  │  Data rate per stream (Mbit/s)  │  Approximate range (m)   │
│            |                                 ├────────────┬─────────────┤
│            |  Bandwidth of 20 MHz            |   indoor   |   outdoor   │
├────────────┼─────────────────────────────────┼────────────┼─────────────┤
│  802.11a   │  6, 9, 12, 18, 24, 36, 48, 54   │     35     │     120     │
├────────────┼─────────────────────────────────┼────────────┼─────────────┤
│  802.11b   │  1, 2, 5.5, 11                  │     35     │     140     │
├────────────┼─────────────────────────────────┼────────────┼─────────────┤
│  802.11g   │  6, 9, 12, 18, 24, 36, 48, 54   │     38     │     140     │
├────────────┼─────────────────────────────────┼────────────┼─────────────┤
│  802.11n   │  7.2, 14.4, 21.7, 28.9, 43.3,   │     70     │     250     │
│            │  57.8, 65, 72.2,                │            │             │
│            │                                 │            │             │
│            │  With Bandwidth of 40 MHz:      │            │             │
│            │  15, 30, 45, 60, 90, 120,       │            │             │
│            │  135, 150                       │            │             │
└────────────┴─────────────────────────────────┴────────────┴─────────────┘

More about Wi-Fi protocols from Wikipedia.

Wi-Fi Direct
Wi-Fi Direct, initially called Wi-Fi P2P, is a Wi-Fi standard that enables devices to connect easily with each other without requiring a wireless access point.
Here the communication is performed at typical Wi-Fi speeds for everything from file transfer to Internet connectivity. Essentially, Wi-Fi Direct falls under the same values presented at the Wi-Fi table (yet again keeping in mind that signal strength and range are the key factors for a good performance).
More about Wi-Fi Direct from Wikipedia.

Bluetooth
While we need to ascertain several influencing factors to determine if Wi-Fi is faster than USB, Bluetooth is with no doubt in last place when we speak about transfer speed. Designed as a proprietary open wireless technology standard for exchanging data over short distances, the maximum speeds are below the two concurrent options:
┌─────────────┬─────────────┬─────────────────────────────────────────┐
│   Version   │  Data rate  │  Maximum application throughput         │
├─────────────┼─────────────┼─────────────────────────────────────────┤
│  1.2        │  1 Mbit/s   │      0.7 Mbit/s                         │
├─────────────┼─────────────┼─────────────────────────────────────────┤
│  2.0 + EDR  │  3 Mbit/s   │      2.1 Mbit/s                         │
├─────────────┼─────────────┴─────────────────────────────────────────┤
│  3.0 + HS   │                                                       │
├─────────────┤  theoretical data transfer speeds of up to 24 Mbit/s  │
│  4.0        │                                                       │
└─────────────┴───────────────────────────────────────────────────────┘

More about Bluetooth from Wikipedia.

Android Beam
Being a fairly recent technology, Android Beam allows users to transfer data between devices with Near Field Communication (NFC) capabilities.
NFC is even slower than Bluetooth (both short-range communication technologies) and is limited to about 20cm of range getting up to 424 kbit/s.
On a positive note, NFC sets up more quickly than standard Bluetooth and the connection between two NFC devices is automatically established:
Technical comparison:
┌──────────────┬────────────────┬────────────────┬──────────────────────┐
│    Aspect    │  NFC           │   Bluetooth    │ Bluetooth Low Energy │
├──────────────┼────────────────┼────────────────┼──────────────────────┤
│ Network Type │ Point-to-point │ WPAN           │ WPAN                 |
├──────────────┼────────────────┼────────────────┼──────────────────────┤
│ Range        │ &lt; 0.2 m        │ ~100m (class1) │ ~50 m                │
├──────────────┼────────────────┼────────────────┼──────────────────────┤
│ Bit rate     │ 424 kbit/s     │ 2.1 Mbit/s     │ ~1.0 Mbit/s          │
├──────────────┼────────────────┼────────────────┼──────────────────────┤
│ Set-up time  │ &lt; 0.1 s        │ &lt; 6 s          │ &lt; 0.006 s            │
└──────────────┴────────────────┴────────────────┴──────────────────────┘

Only relevant aspects to this answer, for full table see Wikipedia.

Summary
Based on the specifications for each method and their respective version, my classification ordered by fastest-to-lowest would be:

USB assuming version 2.0, since 1.0 is way slow and outdated
Wi-Fi / Wi-Fi Direct
Bluetooth
Android Beam