I'm a heavy coffee drinker (either drip or espresso, although I tend to prefer the latter), and I don't use sweetener in my coffee. So for me, things which seem to complement are nutty tobacco flavors (Vermillion River's Kentucky #7 or Kentucky Amaretto are presently my two high points with a cuppa) or creamy sweet flavors (although I'm still searching for that elusive cheesecake or pure bourbon vanilla to go with my morning caffeine intake -- there's been a couple creamy flavors that have shown promise, but none which I agree with strongly enough to mention here).
I'm also a fan of VapeDudes' caramel apple, although it seems to work better with espresso than with drip coffee, and the lingering Granny Smith acidic aftertaste still can clash a bit.
As to finding a true coffee flavor, I'm still on that journey myself. I think it's highly subjective -- tons of people have encouraged me to try flavors which, it turns out, don't taste at all like coffee to me. The other problem I've found is that because I'm using an eGo-T system, the darker coffee flavors gunk up my atomizer and leave behind a burned taste I'm not partial to. I'll find something that works... eventually...
