To be honest, the answer to this really depends on what you would like to do in your free time. We've been living in the mountains for many years, and we still can't tell which season we like the least or the most.
Spring (April to June): that's when nature awakes. Everything is green, colorful mountain flowers popping out everywhere. The clear skies & fresh morning air are amazing! This is the perfect time for beautiful walks along the river, sunny lunches on the terrace and exploring the beautiful villages around.
Summer (July to September): The warmest months of the year. Living at 1500m altitude, this makes "heat" much more bearable. During the day you can expect up to 25-30°C but at night it cools down, allowing for perfect sleep. These months are also ideal for going "high up" in the mountains, spending a night in a hut, climbing on high peaks!
Autumn (October and November): Val d'Hérens is famous for its many Larch trees, so coming here in those months is amazing for the colors. It is also the season for chestnuts, cheese platters and pumpkin soup! A cozy and warm feeling.
Winter (December to March): Winter Wonderland! Located at the foot of 4000m high mountains, the valley turns white and usually stays under its white coat for the whole period. It's the perfect time to go skiing, snowshoeing, cross-country skiing or ski touring. The valley is very famous for endless ski touring and snowshoeing options, there is a 20 km cross-country track in front of the house and the beautiful ski resorts of Arolla and Evolène are only a few minutes away by bus (10 minutes to Evolène, 20 minutes to Arolla)