I worked in the healthcare sector for most of my career. I write about healthcare stocks nearly every day. A significant chunk of my personal portfolio is invested in healthcare stocks as well. My view is that the sector remains one of the best areas to invest in.

Every now and then, someone will ask me what my favorite healthcare stocks are. It's easy for me to answer that question right now, but only if we're talking about investing with a long-term perspective. Here's why my top three healthcare stocks for long-term investors are Guardant Health (GH 13.37%), Illumina (ILMN 0.59%), and Intuitive Surgical (ISRG 1.62%).

Businessman pointing to healthcare icons.

Image source: Getty Images.

1. Guardant Health

Imagine a world where any type of cancer could be detected early with a simple blood test. The prognosis for many patients would be drastically improved from today. Guardant Health isn't just imagining this future, it's working hard to make it a reality.

The company develops liquid biopsies, blood tests that detect cancer by finding tiny fragments of DNA that break away from tumor cells. Guardant Health already markets a couple of products for which demand is skyrocketing. Guardant360 matches patients with advanced-stage cancer with the appropriate therapy. Drugmakers use GuardantOmni to screen patients for clinical studies involving cancer drugs.

I'm even more excited about the potential for Guardant Health's latest product line. The company launched its Lunar DNA tests earlier this year for use at this point only for researchers in detecting early stage cancer and recurrence of cancer. But it's obvious that the opportunity for these tests goes well beyond just research use.

Guardant Health co-founder and CEO Helmy Eltoukhy stated in the company's second-quarter conference call that the total addressable market for its current pipeline of products is more than $50 billion. There's at least a $6 billion market for its Guardant360 and GuardantOmni liquid biopsies. With the company's market cap at close to $9.5 billion, my view is that this stock is still only in its early stages of growth.

2. Illumina

When Illumina first opened its doors in 1998, the human genome still hadn't been completely mapped. The cost of sequencing a human genome cost more than $10 million through 2006. But Illumina's gene-sequencing technology helped slash that cost in subsequent years. Today, a human genome can be sequenced for around $1,000.

Illumina thinks that its latest gene-sequencing platform, NovaSeq, will lead to the cost of sequencing a human genome falling to around $100. This would greatly expand the market for gene sequencing. Illumina CEO Francis deSouza said earlier this year that, in the future, "the ubiquity and impact of genomics will dwarf everything we've seen to date."

I think that liquid biopsies, as well as the broader scope of cancer research and treatment, present a huge growth opportunity for Illumina. In addition, the company is poised to benefit from the expansion of population genomics initiatives where hundreds of thousands of genomes are mapped, from the increased used of noninvasive prenatal testing (NIPT), from higher demand for personal genomics products such as Ancestry and 23andMe, and from a greater focus on researching rare and undiagnosed diseases.

Illumina could hit some temporary bumps in the road as it did in the second quarter. However, I continue to believe that this stock will be a big winner over the long run as genomic sequencing becomes more prevalent.

3. Intuitive Surgical

I remember the days when the use of a robot in performing surgery would have been the stuff of science fiction. Intuitive Surgical turned science fiction into fact with its groundbreaking da Vinci robotic surgical system. There are now nearly 5,000 da Vinci systems installed across the world.

But while Intuitive Surgical has made incredible progress, there's still a long ways to go. Most surgical procedures still can't be performed using robotic assistance. Not all of the procedures that can be performed using a robot actually are done with robotic assistance. This presents a great opportunity for Intuitive.

The company also is poised to benefit from aging demographic trends. As people grow older, they're more likely to require the types of surgery for which da Vinci is already ideally suited. This isn't just a U.S. phenomenon, by the way. Many other countries are experiencing similar growth in their older populations.

One of my favorite things about Intuitive Surgical is its impressive recurring revenue, which currently represents over 70% of total revenue. I look for the company's recurring revenue to keep growing as more customers lease systems and as the types of procedures for which robotic surgery is applicable increase.

What about competition?

All three of these companies face rivals that want to succeed just as much as they do. However, I like the head starts that Guardant Health, Illumina, and Intuitive Surgical claim. It's certainly not impossible to dethrone a leader in a market, but it's also not an easy task. I look for these three companies to remain at the forefront of their respective niches for a long time to come -- and to deliver solid returns for investors in the process.