What decompression algorithms have the shortest implementations?
I expect short decompressors with algorithms that do not maintain an explicit model and use "simple" encodings. An LZ encoding might use (length, offset) with Fibonacci coding, with an offset of 0 meaning a run of length literal codes, (minus) one a repetition of the last code, ….
I expect decompressor size to depend on (possibly abstract) machine - "MS Windows x64", NC4000, …
(The implied model being Things seen before are more likely than anything new, with short, local repetitions more likely than large ones from way back.)
That said, you may be after shortest length of compressed code+decompressor - Kolmogorov complexity.