The new Onyxia raid and VoA are both extremely easy fights to multibox (with ranged DPS). As long as you have the gear to do 3k+ DPS and you have a key set up to strafe left/right to move out of fires, you should have no problem with either of these fights.

ToC is fairly complicated in some parts, but definately do-able.The last boss in the first encounter and the very last boss are the toughest encounters. Icehowl's charge leaves zero room for error. If you don't move fast enough, your whole team (or at least the 4 that are on /follow) will die. Anub's tunnell business is also pretty lethal if you can't quickly switch and run one character away from the rest of your group.

Funnily enough, ToC25 is actually easier to box than ToC10, for a couple of reasons. You're less likely to be charge/tunnel target out of 25 players, and the complicated DPS fights (dispelling/interrupting Lord Jarraxus, multiple forms of CC/dispelling/interrupting during champions) is too much for one person to handle. In 25, however, you can pretty much just DPS and let other people worry about the interrupting and CC. I've only managed to successfully box ToC25 twice, and the first time all my chars got stomped when he picked me to charge. Luckily all my Ankh's were up, and he didn't charge my group again. I've tried ToC10 once and it was a miserable failure. Without a much more mixed DPS (DK to control a melee, rogue to interrupt a healer, X class to focus on dispelling, hunter for frost traps, priest for mass dispel, etc) there's just too much to do during the faction champions encounter.

Anyway, I'd definately suggest that anyone who boxes ranged DPS start farming VoA/Ony (10 or 25, doesn't matter). They both have nice loot and are very easy encounters to multibox.