I would agree about Ambleside. Gives you a good access point to get behind Windermere and head towards the Langdales, Coniston Hodge Close etc. Over Kirkstone via the Struggle and you are at Brotherswater and Ullswater. Rydal, Grasmere and even Thirlmere are on your doorstep and its not too far along the main road to Keswick - hang a left and you are into Borrowdale. A slightly longer trip to Buttermere and Crummock but still no great hassle as you can pick a route with plenty of options en route.
There isn't a best time of year - it is busy all year round these days. In Elterwater a couple of weeks back and the NT car park was full mid week. Buttermere last Wednesday was mobbed with folk parking all the way up the hill. I work on the Steamers on Ullswater and we have been getting over 100 on the 11 o'clock boat recently. 20 years ago we did have a "this time of year" but that doesn't apply anywhere near as much now.
