that's a hard one. consistently i'd now say sven. and richie, who also had a few appearances there over the last few years (cocoon guest sets and amnesia closing parties).
as for one-offs, here are a few which I found outstandingly good...
amnesia closing 2007: richie and villalobos (who were to play back to back) missed their flight and arrived about 5 hours late. this resulted in marco carola, who back then wasn't the big name he is now, playing a 6 hour set instead of one or two. massive. the vibe was also massive when richie and ricardo eventually turned up. last year of the old terrace layout too and my first ever amnesia closing. won't ever forget.
amnesia closing 2009: luciano. last year he really did it for me. and if I remember correctly it was also the year richie then played claude vonstroke's "who's afraid of detroit" as last tune. KILLER moment.
amnesia opening 2013: maceo plex. it's definitely his year (he was also really good at enter two nights ago)