Let me start my answer with my creds: although I've been in nursing school
for the past 2 years and will start an O.R. job in June, I've been a practicing Podiatrist for nearly 30 years, so I KNOW feet...
Not to give you a short answer, but the best shoe is the one that feels best. You won't really know until you try them out in the O.R.
Now, for a more helpful answer...
I've been really happy with Crocs during my nursing school clinicals which included 48 hr in the O.R. in 8 hr shifts. There are lots of cheap copies out there:some are really bad, others are pretty close to the real thing. I have one pair from PayLess Shoes which are terrific. For some odd reason, the real Crocs are not available in white-- closest thing is a sort of tannish.
A good pair of athletic shoes is a great investment. These can be jogging, walking, cross trainers etc. You don't have to buy top of the line for megabucks, but do go to an athletic store that carries a lot of brands and try on many different ones. I like New Balance and Asics mainly because they come in different widths. If you're a woman and have large feet, consider a corresponding men's size: they're often cut fuller.
Make sure the shoes have removable insoles (most of the brand name ones do) and add a pair of Spenco Polysorb Insoles. Check Spenco.com for dealers. These have a neoprene top cover and a nice arch for a near-custom fit.
If you have bad arch problems, an orthotic of some sort would be a big help. Custom ones are most effective, but also most expensive. Check out these: http://www.superfeet.com/default.aspx
I've dispensed the plain black $19.95 ones with great success. The new orange and berry ones are new to me, but they look terrific for nursing and a relative bargain at $40 (compared to $400ish for custom made).