You only get one first impression, so make it a good one.
I'm often asked what I look for in a portfolio of creative work. Students and intern designers ask this question commonly. What matters most in a portfolio is aligning what is shown with who matters most, incorporating personality into the work, and explaining some of the processes used for solving problems.
A good portfolio shows more than just beautiful work
A good portfolio should explain or demonstrate how you process problems.
It's nice to know a designer can use Photoshop or Sketch, but a stronger selling point for employers is to know she can identify the most important parts of the work or problem, prioritize, conceptualize, get and provide feedback, reiterate, and launch.
You can't demonstrate critical thinking abilities with a few flashy photos or a link to a Dribbble profile. Show sketches and notes, and describe some of the ways you like to work within your portfolio.
Take time to explain the process you took to get from ideation to solution, you don't need to do it for every project, but one or two solid examples can go a long way in showing how you work best.
A good portfolio presents the "best” work, as defined by you
Shaz Madani once said that "seeing one bad project can outdo all the good work [in a resume].”
To some degree this is a good point: put your best foot forward. You don't need to show everything you've ever done in your portfolio.
That doesn't mean you hide the work that didn't succeed or was rejected for one reason or another. In-fact: sometimes showing what failed adds a lot to a portfolio (particularly if you share insights into why you think the work failed and what you learned from the experience).
Yes, show your best work, but best in this case doesn't mean most successful or most beautiful; it means the work you're most proud of.
I would argue one point, however, is to not show conceptual work that involves non-clients or partners. A redesign of Google is an interesting thought exercise, but it does not accurately demonstrate how you work through relevant problems. Google already has designers to do that.
Highlight the work you were most excited to work on, that demonstrates the types of projects you like to work on, and can provide clear examples of what you're capable of doing and learning.
A good portfolio is designed around a specific audience
If you try to make everyone happy, you'll make nobody happy. The same goes for your portfolio: you can't give everyone who might be coming to your portfolio everything they want to see. Attempting to do so is arguably a huge waste of your time.
Instead, focus on one or two personas you want to tailor your portfolio to. By aligning what you show and message to target a single audience, you strengthen the portfolio for that particular audience. Making the experience of browsing your portfolio easy and memorable for one type of audience is vastly more powerful than making it dull and painful to explore for everyone.
Focus on presentation for one specific audience. Maybe that means you're building a portfolio for one specific client or potential employer. Or perhaps it's slightly broader than that. Regardless: everything in your portfolio should be directed at the needs of one particular audience, clearly describing how you can provide value to their business.
A good portfolio is honest
It shouldn't have to be said, but unfortunately it seems like it must: be honest in your portfolio.
Honesty in a resume or portfolio does two key things: the first is that it sets you up for success. If you say you're capable of more than you're capable of, then you wind-up getting a work load or project that overwhelms you, that's likely to end in tears. On the other hand: if you are honest about what you're capable of, what you want to be doing, and where you struggle, you're setting yourself up to succeed wherever you go (because everyone will have expectations that align).
The other thing honesty in your portfolio does is ensures you end up working with people who will like you. There's little worse than hiring a new designer and realizing they have a little problem with drinking during client interviews.
Be honest and transparent, let the tone of your portfolio carry some of that weight, but also don't shy away from including a nice summary of your personality and interests.
A good portfolio is reflective
Your work is one thing, but you are the other part of that work.
Recruiters, companies, and anyone else who is looking at your portfolio absolutely wants to see what you've produced, but more important than that: they want to see who you are, why you do the work you do, and why you could be someone they wouldn't mind getting stuck in an elevator with for six hours.
In conclusion, if you want to build a strong portfolio:
Show some of your work process through a blog post or detailed breakdown. I think that Teehan+Lax does an exceptional job of this type of thinking-detail view, as one example.
Present your best work: the work that you get excited to talk about. Maybe it's work that flopped or was never used, but it's work that you are proud of. There's a lot of this already out in the web (take this Dribbble search for example), it's ok to include these works in your portfolio. My only caveat to this is to not show conceptual redesign or product work.
Also, don't be afraid to be you. It's ok to write"fuck,” to show your work in the context of a messy office, or otherwise demonstrate who you are and how you tackle problems…as long as it's honest and relevant to the work.