In my experience, I rate the KOF bosses out of 10 - 1 being easiest, and 10 being hardest...all on Difficulty level 8 of course: -
'94 Rugal 1st Form: 7 - I still find his 1st form hard to some degree...if you mess up the throw tactic, he'll punish you bad...
'94 Rugal 2nd Form: 10 - Sorry but I just find this guy waayy hard...the Taunt trick works to some degree, but still he finishes me off by getting me with his throw. I still haven't beaten him yet. DON'T JUMP at him or you'll be sorry...
'95 Saisyu/Omega Rugal 8 and 9 - VERY difficult. Mostly because Saisyu takes a lot of my life at first. O.Rugal is very hard too, but falls for sweep patterns. Nearly beat him.
'96 Goenitz 4 - Easy. There are 2 simple ways to beat him. You can either 1) Use anyone with a projectile/half projectile (Terry and Mai are good) and stand about 3/4 away from him and throw the projectile. Goenitz will foolishly do his Teleport nearly ALL the time...and when he recovers throw or combo him. Repeat and hes dead. 2) Simply jump over him and land close and immediatley throw him. Repeat and you win easy.
'97 Orochi 2 - HAHAHA He doesn't like it when you roll past him HAHAHA
'98 Omega Rugal 4 - Hmm...be careful. Simply keep jumping up-back in a corner and keep hitting D. He'll try to jump to counter you but he'll get nailed by your jumping D. Repeat. You win.
'99 Krizalid'v99 7 - VERY cheap. Don't play a close slugging match with him. It's suicidal. Instead use Leona and Earring for easy win.
'00 Tom Sellick 2 - The caped crusader is just too easy. Simple run towards him and super jump over him. He'll do the high jumping Slashing thing. Now you run back, when he lands you rape him.
'01 Original Rugal 4 - You only really have one shot with this tactic, and the tactic is when the round starts, knock him over, and when he gets up, IMMEDIATLY do a combo that knocks him down again. Repeat. He will NEVER block. I managed to beat him with this tactic using 4/0 fighters!!
'01 Igniz 5 - Not hard at all. But difficult. Universal tactic is back into a corner, and when Igniz dashes at you, you super jump over him - he'll do either the super blast thing or the Merry Maypole. Now YOU run back and rape him.