A really big thing is budget, and do you want a apartment, studio, or house?
place with night life that is close or in downtown would be SW Portland, or NW. NW would be the best in my opinion, but can also be the highest price, either way though in Portland we have some of the best public transportation and it will get you any where you need to be.
Also SE is closer to the mountain (cheaper houses) about 5-10 min drive to downtown or 10-20 min bus ride.
Give us some budget info, and what style of living your looking for and we can be more specific!