Surprised to see how many people answered Foundry. It is what I use, and I'm a fan, but it has a significant learning curve and you can't buy all the WotC D&D 5e content all prepped, so I thought Roll20 or Fantasy Grounds would be in the lead.
I've used or at least significantly tested a large number of VTTs, but now exclusively run my games in Foundry. For over a year, when I've joined online games as a player, it has been exclusively Roll20 and Foundry. Fantasy Grounds used to be a lot more common.
My 5e VTT journey has been RealmWorks (not a true VTT but allows showing the map with fog of war reveal)--> MapTool-->Foundry.
If I was running a WotC adventures, I would probably buy a sub and the 5e content on Roll20, even though I have foundry, just to avoid the prep time and have smoother 5e support. I would actually prefer Fantasy Grounds for 5e (for the automated to-hit and damage calculations) but connectivity was always a huge issue. Also, I like my players not having to download and install software.
For less crunchy games, I really like Role. Its main focus was to build an excellent Video Conferencing system for gamers with customizable character sheets and die rolling. But they added very easy to use map and token support to show basic positioning or to share maps and pictures. It's really smooth and easy to use.
Role - Every story, every world, every version of you.
For in-person games where I want to display the map on a vertical screen and use minis on it, I still find MapTool to be the most efficient way to do it. And it is free.
I've spent many many hours trying out other VTTs as well, but the above are the only ones I've used to run games in, bought, and/or subscribed to for more than a month.