I"m gonna say Alexi, obviously, but I do have to give respect to Kirk. At least up until ...And Justice For All. While not being the most technical lead guitarist ever, he does have a way of producing some memorable solos. Listen to Fade to Black, for instance. that solo gave me chills the first time I heard it. It builds, and builds, and builds, and comes to this awesome descendiung run that just sounds beautiful. say what you want about Metallica nowadays, but their first four albums are masterpieces, with some awesome guitar work on them.
Not every guitarist has to be able to sweep pick every arpeggio known to man, and not every guitar solo has to be full of sweep-picking.