Felicity, because you know what would definitely be worse than remaining with Felicity?
No one. This show has always been at its absolute best when romance isn't the focus. The show started deteriorating the second the producers decided that audiences wanted to be force-fed melodrama with no end in sight.
You speak as if the problem is the specific character Ollie is being paired with, as opposed to the writers being terrible at writing romantic subplots. They could pair him with Zombie Alt-Universe Black Canary, and the show would have the exact same problems- constant stupid will-they-won't-they, arbitrary lying, nonsensical character reactions, and breakups every half a season.
Just have him marry Felicity and be done with it.