Well, it usually depends on what your forte is. For some people, it might be so hard to script, but easy to map, and it might be the other way around. For me, I find scripting the easiest, probably since I have a good writing ability and I find it easy to memorize all the commands, while I suck terribly at mapping, probably since I have no artistic skill whatsoever.
Here are my views:
Scripting - Probably the easiest for me, since ideas come fast, and then get onto the computer.
Spriting - I have no clue about this, since I 1.) have no artistic ability and 2.) never tried it.
Mapping - My worst ^_^! I can never make a good map (trust me, my first hack was going to take place in the Kanto without any changes!).
Beta Testing - I am a perfectionist. I'd be playing for hours looking for grammatical errors, holes in walls, etc. Too bad I've never beta tested yet :-(