For Red, the Missingno glitches were awesome. However, they can technically still be done for Yellow, you just need a more... Interesting approach. Similar to the Mew glitch, but by battling other stuff.
I think that items can be cloned that way too, haven't tested it in a long while, though.
Gold/Silver/Crystal has the second easiest Pokemon Cloning Glitch, though, IMO.(Given that Gen 1 had a Pokemon cloning glitch, but it involved the Link Cable, instead.) Crystal's was slightly harder to pull off, though, and they all had a higher chance of corrupting data. =P
I honestly wish that the cloning glitches
would return. That's more or less a staple of the series, since every generation but 5 has had
some form of it.(Even D/P, though that was much harder than any of the others, since you needed precise enough timing,
and it didn't even work for everyone, due to fluctuations in routers.)
