Which candidate would be best on foreign policy and why.

[Crumpets]

My answer's probably obvious, but I'll put some numbers behind it so it's not pure hackery:

As Secretary of State, she helped restore the US's credibility abroad. Between 2008 (the last year of the Bush presidency) to 2012 (Hillary's last full year as SoS) the favorability rating of the US went up 13 points in Canada, 7 points in the UK, 27 points in France, 21 points in Germany, 2 points in China, 12 points in South Africa, and 19 points in Mexico.
http://www.pewglobal.org/database/

She launched the Global Hunger and Food Security Initiative, which so far has been a resounding success. This will probably do a lot more good for the world than it will receive credit for, simply because food security isn't as "sexy" of an issue as things like war and trade, but this is big.
http://www.feedthefuture.gov/progress

She helped save a historic treaty between Turkey and Armenia.
http://www.foxnews.com/story/2009/10/10/clinton-helps-save-historic-turkey-armenia-accord.html

Unlike Biden, Hillary supported the raid on the Bin Laden compound.
http://www.politico.com/story/2015/08/joe-biden-bin-laden-raid-defense-hillary-clinton-2016-campaign-121779

And as Senator, she was able to move 80,000 reserve soldiers fighting in multiple deployments in Iraq to the regular Army to ensure they received full care and benefits.
The Buffalo News. July 14th, 2007

[Crumpets]

My answer's probably obvious, but I'll put some numbers behind it so it's not pure hackery:

As Secretary of State, she helped restore the US's credibility abroad. Between 2008 (the last year of the Bush presidency) to 2012 (Hillary's last full year as SoS) the favorability rating of the US went up 13 points in Canada, 7 points in the UK, 27 points in France, 21 points in Germany, 2 points in China, 12 points in South Africa, and 19 points in Mexico.
http://www.pewglobal.org/database/

She launched the Global Hunger and Food Security Initiative, which so far has been a resounding success. This will probably do a lot more good for the world than it will receive credit for, simply because food security isn't as "sexy" of an issue as things like war and trade, but this is big.
http://www.feedthefuture.gov/progress

She helped save a historic treaty between Turkey and Armenia.
http://www.foxnews.com/story/2009/10/10/clinton-helps-save-historic-turkey-armenia-accord.html

Unlike Biden, Hillary supported the raid on the Bin Laden compound.
http://www.politico.com/story/2015/08/joe-biden-bin-laden-raid-defense-hillary-clinton-2016-campaign-121779

And as Senator, she was able to move 80,000 reserve soldiers fighting in multiple deployments in Iraq to the regular Army to ensure they received full care and benefits.
The Buffalo News. July 14th, 2007

Just thought of two more to add:

She oversaw the normalization of relations with Burma.
http://www.bbc.com/news/world-asia-16554415

And she is one of the most traveled diplomats in US history, visiting 112 countries as SoS.
http://www.csmonitor.com/USA/Foreign-Policy/2012/0905/Hillary-Clinton-vs.-Condi-Rice-Who-gets-the-most-traveled-crown