(S01-E09 Derailed) : Chris Bauer as Dr. Theodore "Ted" Bryar
(S02-E08 Empty Planet) : Jamie Elman as Kenneth Roberts
(S02-E13/23 No Way Out/No Way Out ptII: The Evilution of Frank) : Keith Carradine as Frank and Amy Madigan as Jane (She may not have played the unsub but her performance was definitely note-worthy)
(S02-E14/15 The Big Game/Revelations) : James Van Der Beek as Raphael / Tobias Henkel
(S03-E02 In Birth And Death) : Eddie Cibrian as Joe Smith
(S03-E10 True Night) : Frankie Muniz as Jonny McHale
(S04-E03 Minimal Loss) : Luke Perry as Benjamin Cyrus
(S04-E08 Masterpiece) : Jason Alexander as Prof. Rothchild
..and many more I'm sure..
Which one(s) stand out for you???