A good boxer. Sluggers and swarmers will give you a chance to work. Losing to a Floyd Jr, Whitaker, or Hopkins does damage to your pride.
Mayweather and Hopkins. Both are expert spoilers and defensive wizards. These guys don't as much go all out to win but all out not to lose. If they decide not to stand and fight there is little you can do about it. No one minds if you lose in a fight of the year thriller, but these guys can make even the most exciting fighters look bad.
I wouldn't even spar with Mike, even if it meant trying to grab a few mins of fame, unless I was getting paid, cause you know he would be trying to knock you out, that he probably would knock out to, as these is no hiding place in a ring if you cant hurt the guy, he also treats sparring like a real fights from what I've heard.