I voted for Spike, but in retrospect, Wesley may have been a better choice. As Nifft said, Spike really breathed life into the final season of Angel, and I loved him on Buffy. In fact, the fifth season of Angel is really the only one that I liked on Angel- the only one I'm buying on DVD, certainly- and all because, IMO of Spike and the chemistry he brought to the show.
That being said, I have to agree that Wesley is probably the best choice, if only because he was there all the way through on Angel- all five seasons- and he showed the most growth of any character on the show. His character arc, in fact, is probably one of the most growth and change that I've seen on any show- with the possible exception of Spike, of course, who underwent the vast majority of his on Buffy. Everyone else was rather stale, including (sadly) Angel, though he had some really good moments.
I did like Amy Acker as Illyria, though I absolutely hated her as Fred. Gunn was just... bleah. Cordelia was a far more interesting and deep character on Buffy than she was on Angel, and Connor was just... ugh... though I did kind of like him on the last episode.