The best core i5 looking at the basis of price and performance than in the ivy bridge lineup it's got to be the core i5 3350P .It's only 177$ and that price it's very good.It does not have an integrated gpu but i assume you were anyways going to use a discrete graphics card.If you want the integrated graphics you could get the i5 3570 .It's a great quad core processor and only lacks a fully unlocked multiplier.
From the haswell lineup you could get the i5 4430,it's fast and also very cheap.If you want to overclock then your work becomes much easier since you have to choose between only 2 cpu's ie:the i5 3570k or the i5 4670k.Simply when compared the i5 3570k will overclock better than it's haswell counterpart but i5 4670k will simply compensate for it by still being faster than it at a lower clock speed.