In general or for a wedding?
For a wedding.. we did the french terrace and loved it - great view. I think we started at 8, illuminations was at 9 or 9:30 - We just had desserts. Everyone loved it, well worth the money.
In general - there are a few places with good views. The benches by canada, the tables by the outdoor cafe in mexico (closest to the water .. and these fill up fast) - the bridge by mexico.. the bridge by france .. the french terrace if it is open to the public, and even in the front of the world showcase, between the two shops. Basically, anywhere you can get close to the railing or bridge and will give you a clear view. As for when to get there.. it depends both on the time of year and your tolerance level.. Busy times of year, folks start camping out about an hour beforehand.. New year's eve about 2-3 hours before hand. Less busy times, 10-20 minutes and you could still find a good spot.
G/L
--Heather