Sidebottom has played just 16 ODIs and is in no way a certainty for the Windies ODIs so I would put Anderson above him. Broad is a far better cricketer than Anderson and is almost certainly going to become a better bowler, but at the moment I would still rather have Anderson bowling than Broad.
Few ODIs though Sidebottom has played, he simply strikes me (and I'm surprised he'd not strike anyone TBH) as being possessed of far more basic skills than Anderson - ie, his lines and lengths are so much better it's untrue. He also looks to me to have more of a clue about what to do when a batsman looks to really take to him. Sidebottom certainly isn't a proven ODI bowler, far from it, but he does look a potentially good one. Anderson, in my view, doesn't and never has.
As for Broad - in England's last 25 ODIs (ie, since October 2007), Broad's figures read economy-rate 4.85-an-over, average 23.85. In the same time, Anderson's (he was dropped for the most recent one) are economy-rate 5.56-an-over, average 65.57. I just can't see how Anderson can be considered > Broad in the ODI game currently.
As for Dimi, I would have him in every England ODI side and would have done for the past 4/5 years, but I think his job in the England side would be to bowl 10 overs for < 40, and if he picked up a couple of wickets along the way, it would be great. I don't see him as a bowler who could single-handedly win an international game.
Mascarenhas would indeed be good for such a job and probably not very often much beyond it. However, pretty well no bowler can ever single-handedly win a one-day game. When you're limited to 10 overs, taking the five- or six-for required to do that is a once-in-50-games or so occurrance, for all but the very, very best bowlers. I reckon that isn't beyond Mascarenhas but clearly he'd be likely to do it less often than an Anderson would. However, he'd also be likely to go for the 10-58-1-type figures far, far less often. Ergo, he'd play a large part in losing far fewer games. Ergo, to me, he'd be a much better bowler.
That, however, is an if. Mascarenhas' bowling form was poor in 2008 - regularly went for plenty - and he needs to rediscover his 2007 form before I can get my hopes up about him.