What's the fastest I don't know, but I know what's the most readable - that what's shortest and simplest. Even if it's a little bit slower than other solution it's worth to use.
So use:
"string".replace("a", "b");
"string".replace(/abc?/g, "def");
And enjoy good code instead of faster (well... 1/100000 sec. is not a difference) and ugly one. ;)
