Best time of day to go truly depends on your Epcot day plan. If you are a foodie and into fancy champagne/drinks and small bites, you might want to consider the fireworks package. I haven't done it, as that's not my thing and it's pricey, but you might love it!
If that's not your thing, I recommend going early in your Epcot day. We had an ADR at Geo-82 @ 12:00 and it was perfect! We had two very well-made cocktails and a cheese board, with a perfect view, and that was a great way to start our Epcot day! After Geo-82 we leisurely strolled around WS with a few glasses of wine and took in some QS bites as well as all the sights and sounds! We were not there during a festival time, but if you are you will have plenty of food/drink options after Geo-82. Enjoy your trip!