For people interested in getting into the market but don't have the time to navigate a bunch of different options (or just hate dealing with that kind of stuff), I've recommended a robo investor like Betterment. It charges a (very low) percentage fee so it's definitely not going to be the most cost effective option, but it's engaging and the tools are cool. Even though I now have most of my money elsewhere, I have the accounts balances linked to Betterment and use it all the time to guestimate how I'll be looking 5, 10, 20 years down the road.
After you get a little more comfortable (or if you want to just skip to the "intermediate" low-risk investing) Vanguard is probably the gold standard buying VTI or VOO.
That said, like SD is getting at make sure you understand all the tax-advantaged options available. HSA is probably the sneakiest one if you have that through work.