Re: LIAN HARPER?!
Interesting question - I like him single, but my favourite relationship for him was definitely Starfire. It was a great "chalk and cheese" pairing, the uptight overachieving kid and the exotic, passionate alien princess; she showed him it was okay to express your feelings and be your own person, he showed her that there could be more to life than simple passion and reaction.
It also moved him out of the Batclan, which is where I prefer him. Always welcome for a visit, but not living there anymore. He's grown up and moved out of his Dad's place. I liked him in NEw York.