It's hard to come up with an objective measure of which candidates are being attacked the most, but this ought to be a reasonably interesting proxy.
I looked at the press releases from five sources: the Clinton campaign, the Obama campaign, the McCain campaign, the RNC, and the DNC, and counted the number of times that McCain, Clinton or Obama was mentioned in the headline of the press release. (For Obama press releases, which tend to have vague headlines like "Barack Obama Statement on Iran", I also counted hits in the press release abstract). Then I sorted the hits by the month of the campaign from September onward.
These figures were tallied by hand and so may be slightly imprecise, but you should certainly get the general idea. Also, this should be obvious, but the idea was to account for attacks only, so I didn't count instances in which say a DNC press release mentioned Clinton, or a McCain release mentioned McCain himself.
Let me also give you that data in tabular form, and then a few observations.
1. It's manifest that the big break in the Democratic campaign came in February. Obama took just 10 incoming hits in January, but 51 in February, as both the RNC and the Clinton campaign significantly ramped up their efforts against him.
2. Clinton's incoming hits peaked in January, and have since dwindled basically to nothing. She hasn't been the subject of either an RNC or a McCain press release since March. Overall, since March 1st, Obama has taken 151 incoming hits, McCain has taken 144, and Clinton has taken 9.
3. The Obama campaign does very, very little attacking (quite possibly too little), at least in the form of press releases. That doesn't mean that they won't go negative, but they prefer to wait for an opportunity to counter-punch and/or to do so somewhat surreptitiously. But what they won't usually do is to try and dictate the course of a news cycle with an attack.
4. In contrast, the Clinton press shop is always operating at a fever pitch, and much of that involves attacking their opponents. During March and April, the Clinton press shop was hitting Obama nearly once a day. But the Clinton campaign has also delivered considerably more hits on McCain than the Obama campaign has (at least through its press releases). Also, note that Clinton has considerably cut down on her hits on Obama for the past several weeks.