I'll go against the grain of most answer and recommend Knuth's MMIX variant of the MIPS RISC architecture. It won't be as practically useful as x86 or ARM assembly languages (not that they're all that crucial themselves in most real-life jobs these days...;-), but it WILL unlock for you the magic of Knuth's latest version of the greatest-ever masterpiece on deep low-level understanding of algorithms and data structures -- TAOCP, "The Art of Computer Programming". The links from the two URLs I've quoted are a great way to start exploring this possibility!
