Compared to what? If you've never done a wedding before, it also has a bunch of unique attributes. My angle was based on an approach of someone that had never done 'x' event before .. and learning to be proficient with it. If you've never done club work before, it could be difficult as well. Same with Karaoke, etc.
As long as the operative word is "DJ" then they are all equal.
The time to convince people of your skills and credentials is in the SALES process. Fulfillment of the gig is always as simple as delivering whatever it is you sold. You can always enlist help or employ people as necessary to get a contract fulfilled. (i.e. If you're not a great emcee or dance instructor - hire one)
There is little you can do as a DJ to compensate for an up-front inability to effectively communicate your potential unless someone else is representing you.
I've done all seven, and more. Each has it's own focus and goals that need to be understood, and if the client does any due diligence at all - they will know before signing the contract if you are up to the task.
For me it would be Bar Mitzvah simply because I've never done one and from what I read there is alot to remember....I've done all the rest on a regular basis over the years but it would take a while to get back in to the club stuff if I ever did it again
Like the others said the one that would be the hardest would be the one you are least used to doing