I'd say Alistair because it's a quite well written ark, and while I like both Leliana and Morrigan, the former seemed a bit too... well, easy and accessable, and the latter was a bit temperamental, although a very interesting character. In terms of Zevran, by the time I usually picked him up, I was usually already pretty involved with the others to fully dedicate time to the process.
That said, I prefer the romances in Dragon Age 2, even if they were a bit harder. Merill was a bit more interesting as a character than Leliana, Fenris was plain awesome, and Isabella and Anders look interesting even though in the end, Anders is going to die no matter what. I just wish I was allowed to romance Aveline.