Daren Ganga is actually a pretty good batsman who simply can't hack international cricket. Someone like Xavier Marshall however is, as mentioned, simply absolutely hopeless. I'd agree that there's no point counting the likes of those who played for West Indies Third XI (especially some 36-year-old who only played to captain), and also obviously there's no point counting Zimbabweans and Bangladeshis else we'd have thousands - a couple to come to mind are Anwar Hossain Monir and Shadab Kabir.
I'm less than sure about Sreesanth, Gillespie and even Mohammad Sami. Sure, they're all well below international standard in both formats (I'd disagree with the criteria of having to play Twenty20 Internationals BTW, because there are relatively speaking so precious few of them) but I'd not say any of them were obviously diabolical - the latter two even earned their go to some extent at domestic level.
BTW Ian Salisbury being suggested is also not fair to my mind. Again, sure, below Test-class and absolutely dreadful figures but certainly nowhere near as bad as some who have played - he at least earned his go.
Of bowlers, I'd say the worst this decade is probably Richard Dawson, though Adam Sanford is certainly also worth a mention.
And someone else I'd mention, though he doesn't qualify as he (mercifully) never played Test cricket: Darryl Brown. Played a few ODIs for West Indies in 2001/02, despite never achieving anything much in a territorial career of, IIRR, about 3 years. Almost as diabolical a selection as Darren Pattinson, and undoubtedly a far, far, far worse bowler.