I only recommend HASWELL (4th gen) for new builds. Which CPU completely depends on the gaming budget. It's very roughly like THIS:
$800 to $1600: i5-4690K
$1600+: i7-4790K
There's several factors involved including budget, length of time to keep PC, whether VIDEO CONVERTING is a factor, etc.
I was going to mention an i5-4460 or similar but after looking at the current pricing it's hard to recommend (unless a sale). For example, the i5-4460 only has a max Turbo of 3.4GHz whereas the i5-4690K defaults to 3.9GHz and can overclock further if needed.
Price wise it's $220 vs $183 so a $37 difference there for a system that can easily overclock to 40% faster if need be though. On a $1000 computer build that's just under a 4% cost...