VS Hogan, Smackdown 2002
VS The Rock, Summerslam 2002
VS Undertaker, Hell in a Cell No Mercy 2002
VS Angle, Wrestlemania 19
VS Angle, Ironman match, Smackdown 2003
VS Benoit, Smackdown 2003
VS Eddie Guerrero, No Way Out 2004
I'll also go for Lesnar vs. Taker from No Mercy 2002 and Lesnar vs. Eddie from No Way Out 2004.
I really enjoyed his WM rematch with Kurt Angle at Summerslam 2003 as well. The Lesnar vs. Angle vs. Big Show triple threat match from Vengeance 2003 is also worth checking out, it's a pretty wild match. I think Lesnar and Benoit had a really good match on Smackdown near the end of 2003 as well that might be worth tracking down.
Lesnar vs Benoit -SD 2003
Lesnar vs Angle -Ironman match
Lesnar vs Angle -Summerslam 2003
Lesnar vs Angle -WM 19
Lesnar vs Taker -HIAC
Lesnar vs Taker -Biker Chain match
Lesnar vs Hogan -SD 2002
Lesnar vs The Rock -Summerslam 2002
Lesnar vs Guerrero -NWO 2004
His Cell match with Taker was one of the best matches I ever saw. Two superheavyweights not giving a shit and just dropping bombs on each other inside a steel cell. Blood, steel steps thrown, Undertaker tossed around like he was nothing, Taker somehow playing the ultimate sympathetic babyface.