I'm a big fan of dictionaries which give their definitions in plain English. That way you don't end up having to look up 3 words to get the definition you're after. Collins makes some good ones for this. Collins School dictionary for more common words. Collins Student or English dictionary for less common.
It depends whether you want US or UK English. For UK English I reckon the Oxford. I have one I actually bought in Oxford, which was a completely ridiculous thing to do on a backpacking holiday, and I use it often.