Safe Firmwares

This page is meant to compile a list of safe firmwares applicable for each particular exploit by system. If different firmwares support different exploits or features then that should be noted here as well. Firmware numbers are color coded like so:
 * Not Exploitable
 * Hypothetically Exploitable
 * Partially Exploitable
 * Exploitable

Nintendo Wii U

 * 5.5.1 : Kernel exploit for 5.5.0/5.5.1 was leaked on 5/3/16
 * 5.5.1 : Hykem (who has since disappeared, supposedly arrested) claimed to have a working IOSU exploit that would be compatible up through 5.5.1 . Smea released some tools for a hypothetical iosu hack, so when one comes out, there should be a solid base of tools to work with.

Nintendo Wii

 * 4.3 : Highest supported version that can be hacked using the LetterBomb exploit. (This is also the current highest system firmware version.) LetterBomb doesn't work in vWii on the WiiU, so you'll need an exploitable game.
 * 4.2 : Highest supported version that can be hacked using the Bannerbomb exploit.
 * 4.0 : Lowest supported version that can be hacked using the Bannerbomb exploit.

Nintendo 3DS
See https://github.com/Plailect/Guide/wiki for instructions, https://www.3dbrew.org/wiki/3DS_System_Flaws and https://www.3dbrew.org/wiki/3DS_Userland_Flaws for references
 * 11.0 : Userland only; cannot downgrade
 * 9.3-10.7 : Userland, also can downgrade to a more exploitable firmware using an escalation flaw
 * 9.0-9.2 : Fully hackable using a kernel flaw.
 * sub-9.0 : Anywhere from easily and fully hackable to only fully hackable by updating; refer to https://github.com/Plailect/Guide/wiki/9.2.0-Update for instructions to get on the standard version used for hacking
 * 7.0 : Not relevant to exploits, but marks a change in the method used for save encryption. Take note that if your SysNAND firmware is below this number it will result in save files that are incompatible with systems with a SysNAND firmware above this number.  (And vice versa.)  Save corruption is possible if you carelessly swap a game or cartridge between systems without paying attention to this limitation, or downgrade a system from above this number to below it.  Note that only games released after the new encryption was implemented are affected.
 * 4.5 : Highest available firmware for DS profile hack. (Gateway blue card exploit.) Gateway released a downgrader for the Settings app, which lets you install the DS profile hack on an N3DS, though still requires a copy of Cubic Ninja or Ocarina of Time to run the downgrader.
 * 4.1 : Lowest available firmware for DS profile hack. (Gateway blue card exploit.)  If your firmware is below this version you will need to update using a legitimate cartridge with firmware 4.1 - 4.5 included on it.

Old/New differences
The differences between process and availability used to be much different between O3DS and N3DS, but most of the differences these days are relatively small and are limited to downloading different files or (in the case of setting up a9lh) a moderately different process. Refer to Plailect's guide for current instructions and note that a9lh is optional.

Nintendo DSi

 * 1.4.1 : Highest supported firmware for sudokuhax

PlayStation Vita

 * 3.60: Highest available firmware for HENkaku
 * 3.50 : Highest available firmware for PSP mode hack. As of now only the PSP mode of the Vita can be hacked, and doing so requires buying a vulnerable game.
 * 3.36 : Highest available firmware for Live Area Bubble creation and webkit exploitation
 * 3.18 : Highest available firmware for Package Installer usage

PlayStation 3

 * 4.70 : Highest currently available CFW. This is also the highest system firmware version currently available for this system.
 * 3.55 : Highest available stock firmware that CFW or homebrew can be installed on natively. Higher official firmwares can only be downgraded using a hardware flasher, and this is only possible for certain models.

PlayStation Portable

 * 6.61 : Highest available firmware and compatible with custom firmware installations

Xbox 360

 * 2.0.17349.0 : Highest available version and compatible with Reset Glitch Hack