Tough call! So much depends on what you're coming to her for. If you want the book that's often considered her 'gateway,' even though that feels like a weird word for her, 'Mrs Dalloway' is probably the standard recommendation. It's a single day in London, following Clarissa Dalloway's party preparations and the parallel story of a shell-shocked veteran. It's got that famous stream-of-consciousness flow, but it's anchored by a fairly straightforward event. You can feel her experimenting with time and memory without it getting as abstract as some of her later stuff.
I tried 'To the Lighthouse' first and honestly bounced off it hard. The first section, with the Ramsay family at the vacation house, felt like walking through thick fog—beautiful, but I couldn't find my footing. I came back to it after 'Mrs Dalloway' and it clicked; the second half, 'Time Passes,' is maybe the most stunning writing about loss and decay I've ever read. But yeah, starting there can be a rough ride.
Maybe just pick the one whose premise grabs you? Party day, family holiday, an artist's life ('Orlando' is wild and gender-bending but playful), or a literal wave crashing over you ('The Waves' is pure, challenging poetry). No wrong answers, just different levels of immediate accessibility.