Chrome as it has excellent dev tools and a large market share. Then manually check in Safari, Firefox, iOS Simulator during development. Then BrowserStack for quick spot checks. Then check on real iOS/Android/Windows devices in our device lab as emulators are bad at replicating the true user experience (animations, click areas, godawful plastic trackpads on cheap Windows laptops, etc).

Cross-browser testing is hell and responsive has made everything multiple times worse for the amount of manual grunt work that has to go into every non-trivial custom web project. Greatly envy non-web developers who can build an interface once and it's done.