When launching a new product, what sales assets are a must have to make the launch a success?

First the failure mode (for contrast):

PMM does a kick ass job with product decks and slicks. There is a training session where some people seem to be paying attention, but most people are distracted by their day-to-day job of sales. Then when a sales person gets an opportunity, they ask the PMM or PM to come in and help. Or worse, the sales lead complains at the company QBR that her team is not enabled properly. 

What I think is better:

Start with what's in it for the sales person... Is it higher deal value to satisfy quota? Higher win rate?

Then, think through how your sales people will sell the new product... will the SE go and do a demo during the next customer QBR? Will they be able to talk about a broader company story during the next renewal or new customer negotiation? How will they price the new deal with the new product in there?

Then work backwards from there to figure out what assets are required. Often I find that pricing proposals, an updated company/platform story, demos are critical.

Regardless, the key is to have your best sales person (someone everyone in Sales considers to be the best amongst them), to get up on stage and talk about how she will sell the new product. That means, your job is to work with that person and prep her with the right information and assets. 

You never want a marketing or product person to tell how a sales person should do their job. Instant credibility loss and guards come up. Just like you hate a Sales or Product person telling you how to do Marketing. 

Internal training materials/deck

Education should always be a big part of launching the product. The first thing you need to accomplish is getting the sales team to actually care about whatever it is that you're launching. Try not to make this overly academic, make sure you're getting the point across as to what the opportunity is for the sales person to make money. 

Beta/early adopter case studies

I always try to avoid launching products without a couple of well produced case studies from early adopters/beta users. 

Leave behind materials

Could be a deck, a one pager, something that you can leave with the customer after pitching. 

Relevant Personas

This may not always be relevant, but if you're going after a new segment, decision maker, user etc.. It's important for the sales team to be able to conceptualize and empathize with the person they're going to be selling to. 

Video

I'm a huge believer in video. Video content gets more engagement than any other type of content. You can get more information across in a well produced two minute video than you can often do more damage with a two minute video than a long ebook or whitepaper. 

Web Content

Gotta have a good representation on the product page!

There are of course more, but I agree with Steve that it's better to wait a little after the product launches to see where the needs are. For example, 

This will vary depending on how important a given product is as well as its degree of complexity, but for a decently robust feature/product I think you'd want some of the following as a minimum.

[Internal] Product one-pager: Succinct asset that explains what customer pain points we're solving for, what the product does/how it solves them, the value prop and top benefits of the product, how it's different from what competitors have, the pricing and packaging guidelines, and any other product-specific reference information your sales team would find helpful.

Proof points and testimonials: Your prospects want to feel confident that this product works and works for people solving the same problem. If you have any data on the efficacy or ROI of the product, make sure your reps know it! Same goes for customer testimonials -- see if you can get some great soundbytes from participants in the pre-launch product beta that can help de-risk this product for an uncertain prospect.

Collateral: Your reps need a visual asset that helps them speak to the product and helps the prospect have a reference asset that they can use to refresh their memeory on the value/how it works after the sales conversation is over. This is doubly important if the prospect needs to champion your product to other internal decision-makers. Help them tell the story.

Product demo: Either a recorded asset or a talk track. This should really focus on the pain points, value prop, and how the product works to deliver that value prop. It's important to keep the "how this works" part as focused as you can on the value prop and ideally the overall demo on the shorter side so that you don't lose people's attention/they get lost in features instead of what value the solution delivers. 

FAQ/objection handling: Use a beta to understand what types of questions, concerns, and objections come up from someone being introduced to the product for the first time. Then create a simple reference resource that preemptively addresses them; make sure to continue updating this after the launch as you continue to get more feedback from the market.

I agree with everything Courtney wrote and would add that the sales materials need to train the sales team on a compelling event that will drive interest in the product. This could be externally driven or something you define yourself; you need to articulate how you are creating demand. I don't produce all the assets Courtney lists at launch and instead reserve some time in the weeks following the launch to evaluate how it's doing and hear sales feedback. If I need more materials, I will produce them then.

Product one sheets/handouts/leave behinds, strategy/copy for sales outreach email campaigns, internal one-sheets for sales calls/scripts, a demo video, buyer's guide, case studies or testimonials if possible, website page that describes product.

You need to have a slide on the new product and how it fits to your overall story. In cases where the product is a huge release, I would include a one pager. Included in all of the assets are talking points, training, and proof points TO SALES (i.e. a top sales person has used this asset and it worked and why it worked) 

That is best answered by buyers and not by sales people. Go talk to potential buyers and find out what they need to help them make a buying decision.

If you don't have time to do that, make sure you ask that question in your Win/Loss interviews.

The ones that compel a buyer to act. Unless you are a customer of your own product, you can't determine what those things are. You have to learn them from buyers in your target market.