Are we talking overall skill, or skill in a specific area.
For example Nightwing is a more skilled acrobat and gymnast than all of them.
Tim as a Robin is arguably a more skilled Detective and programmer
Red hood is a more skilled Shooter.
Bruce has a a wider range of skills he's more skilled in combat.
The greater skill some have in some areas makes their overall skill higher. For example Dick's acrobatics improve his h2h combat.