A couple of favorites are quality conventional porro prism 7x50's and roof prism 8x42's (inline style) . Both sizes have great properties. I tend to avoid 10x binoculars, as it's much more difficult to keep a steady image when hand-holding them.
Quality 7x50's have a nice large exit pupil, resulting in excellent usefulness at night. Have a modest power (7x) and 50mm lenses, to gather lots of light and a nice field of view. Great in all around use, and especially easy to use.
The 8x42's are lighter, inline, so they are compact, have excellent eye relief and also have a great field of view. 8x42's are often used by bird watchers, looking for detail.
Other sizes tend to have limitations that I find less than optimal.