Transcript of course. They need to look at each of your individual marks and the credits you accumulated and achieved. Of course, ones you failed, or dropped will be seen as well. They will look to see if you fulfill the prerequisites needed for the program you are applying to. The diploma isn't of much use when applying for college as whether or not you graduate from High school will be shown on your transcript anyways.
They will ask for your transcripts I'm pretty sure they can get all the necessary information from them. I wasn't asked for a diploma when I applied.
Transcripts and Diploma have them handy and good luck :D
In all probability a transcript more than a diploma.