Honestly? August. No contest.
I've been taking groups from Delhi to Valley of Flowers for years and August is when the valley is truly alive wildflowers everywhere you look, including the rare Brahmakamal that blooms nowhere else in the world.
July is beautiful too but the roads can be tricky with monsoon landslides. September is peaceful but you're catching the tail end of blooming.
If I had to pick one specific window first two weeks of August. Weather is manageable, flowers are at their fullest and the trail feels magical rather than crowded.
One practical tip if you're coming in a group, book your
tempo traveller in Delhi at least 3–4 weeks ahead. August is peak season and vehicles get booked out fast.
Worth every bit of planning. The valley will genuinely take your breath away.