Investing isn't something that should require continuous effort. And you shouldn't need to check on your stocks every day. When you put quality stocks with solid fundamentals in your portfolio, you can just buy and forget about them. 

Thermo Fisher Scientific (NYSE:TMO) and Costco Wholesale (NASDAQ:COST) are examples of two unstoppable stocks that fit that mold and are likely to continue growing over the years. They are investments that you can buy and hold for not just years, but likely decades.

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1. Thermo Fisher Scientific

Thermo Fisher makes many types of products for the healthcare industry, including laboratory equipment, supplies, and instruments. The business has grown organically and through acquisitions over the years, with sales of $32 billion in 2020 rising more than 76% from the $18 billion the company reported just four years earlier in 2016. And during that time, the company has consistently netted at least 10% of that revenue as profit.

The company recently issued its third-quarter earnings report. For the period ended Sept. 30, sales rose 9% to $9.3 billion -- with 7% of that related to organic growth. For the full year, the company is projecting sales of $37.1 billion, up 15% from 2020, and adjusted per-share earnings of $1.30, up 20%.

Even amid the pandemic, Thermo Fisher is proving it can deliver strong results. It has developed new testing solutions (its COVID-19-related revenue in Q3 was more than $2 billion) which have helped the business not only avoid a drop in sales but generate growth. That adaptability and innovation is key for investors who want to buy and not worry about current industry conditions.

This healthcare stock has been a great buy for investors, rising more than 1,100% over the past decade (vs. 268% for the S&P 500). And there's no reason to think it won't continue delivering impressive returns for investors for the foreseeable future.

2. Costco Wholesale

Costco is another stock that has been doing great regardless of the pandemic's stage. The big-box retailer reported its year-end results for fiscal 2021 (ended Aug. 29) and sales rose an impressive 17.7% to $192 billion. That was on top of the 9.3% revenue growth the business achieved a year earlier when it was already starting to see a surge in demand, driven by the pandemic.

What's remarkable is that even now with the economy opening back up, Costco continues to generate impressive results. The company reports its sales numbers on a monthly basis, and in September revenue was up 15.8% over the year-ago period. In October, revenue rose by 19.2%. The business is proving to be literally unstoppable, continuing to generate double-digit revenue growth in the U.S., Canada, and international markets. 

What this suggests to me is that the company picked up new shoppers amid the pandemic who liked what they saw, and that's contributing to the growth on top of already strong numbers in the previous year. It's a great sign for investors and is a reminder as to why Costco makes for a solid long-term investment.

The only thing that could be better are its margins; Costco generally collects a profit margin of no more than 3% on its revenue. But it has done that consistently over the years. And on nearly $200 billion in revenue, that's still a fairly large chunk of change.

Costco is another top growth stock that has easily outperformed the S&P 500 over the past decade. Its returns during that period top 495%. And it too looks to be a safe bet to continue outperforming the index in light of its consistently strong sales numbers.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.