Keeping it simple - Magpul Industries AR15/M4 BEV Block

The Magpul BEV block tool can be used in place of all your other bench devices. If you only purchase one, this is the one. Easy to work on lowers but flip over and you can do the upper receiver.
The only issue is when working on upper, you really need to put a bolt assembly in and secure it to the block to keep everything stable.