I run Ad-block Plus and NoScript and I've never seen a YouTube ad.
Try ABP first to see if that works. NoScript is a kill-a-fly-with-a-sledgehammer kind of solution that takes some time to get configured to your taste, since it blocks a lot of stuff you probably won't want it to, at least right out of the box. Works like a dream once you teach it what you don't want disabled, though.
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