(I know that he is brave and undefeated but I think Connor is stronger than Groo)
In their kind-of fight, they make Groo out to be quite a bit inferior to Connor.
Are we talking who would win in a fight or who could lift the most weight?
Either case, Buffy's #1, it's her franchise, and she's beaten all of the contenders in proper fights.
#2 would already be different based on the rules. I have no doubt Faith is made out to be physically stronger than Angel, but I think Angel would win in a fight.
STRENGTH:
Buffy
Faith
Angel
Spike
Connor
Groo
WHO WOULD WIN:
Buffy
Angel
Faith/Spike
Connor
Groo
Even then, it's tricky with the three heavy hitters.
Buffy would always win, it's her franchise.
Spike on his best day would beat Angel on his worst day.
Spike on a good day would beat Faith on a bad day.
Faith on a great day would beat Spike or Angel on a lousy one.
Angel & Spike are tricky. Slayers' main advantage seems to be that most vampires are poor fighters. Those two have Slayer-level skill, near-Slayer level strength, and far better durability than Slayers. They're even slightly sunlight resistant, where other vamps dust from the sun instantly, both those guys easily survive temporary exposure.
Regarding Illyria... given time to prep, the heroes would definitely take her down--probably more Giles, Willow, and Wesley than the better fighters. But with no prep, wow, even Buffy, Faith, Angel, and Spike, they couldn't phase her combined. The original attack on Illyria: Angel, Spike, and Wesley led, would've killed Buffy in two seconds. Illyria barely noticed it.