I'll just go through a general break down of each character with my opinions and let you decide.
1) His Quincy costume is something you'd probably have to make yourself. It's a pretty plain, straight-forwards costume, though, and probably wouldn't be too difficult to sew for a beginner with some basic knowledge of sewing.
2) To do his hair accurately, either you'll need long bangs or a wig.
3) The frames for his glasses are pretty unique from what I've seen. A friend of mine used clear sunglasses in a similar style, but it didn't quite look right. It could potentially get really frustrating trying to find the right style of frames.
1) If you do his school uniform, he doesn't wear a tie, so that's one less thing to worry about. His other outfits are ether pretty simple to make (like the white one with the red stripe) or find (loud "Hawaiian" shirt).
2) Again, if your hair isn't long like Chad's, you'll need to get a wig.
3) If you want to make Chad's arm, it may be difficult depending on what you're skilled at working with.
4) This is a minor thing, but I suppose it's worth mentioning. If you're going for more true-to-life cosplay, you might want to consider your height, build, and melanin level and go with whichever character is closest to you. If you don't care, then don't worry about it.
Overall, I'd say that Chad is potentially the easiest. Buy pants, either make or buy a shirt, get a good wig and you're done.