I usually like to throw out the book and stand backwards. Your opponent will never expect it! Element of surprise my friends...element of surprise.
Standing angled does not make you harder to clinch. Normally guys drrop and hug your waist, or clinch when they have their back to something and you're coming forward. It makes minimal dfference. On the other hand, a squarer stance allows you to throw ******* uppercuts when someone tries to grab you. Besides, from a pure boxing standpoint, if they want to clinch, let them.. they'll be getting points taken off.
Depends more on you than the stance. There is no "Best" in Boxing after everyone is dedicated and knows the basics. There is only the "Best for you". If you have freakishly long arms, good power, and only decent speed? A square stance could be great. "Safe", but able to get your offense off while making the other guy waste punches (Clottey, Wright). If you have a strong build, shorter/lower dimensions, devastating power in BOTH hands? A square stance is probably ideal (Tyson). If you are fighting a guy that is fast as hell? You might square up a bit and turtle up to get in and/or counter. Regular (SLIGHTLY angled) if you have a good speed and devastating power/timing in your rear hand. If you are a more timid fighter and/or a counter puncher than needs to exit situations often. Having the hips a bit more angled lets that rear leg kick out and give you space fast. (Some versions of Mayweather, Toney) It is all up to the fighter...
As long as your feet are angled, situation will dictate whether you square your body or keep it closed.