Strength, man. I wanted to be the stronkest of them all.
Edit: Not sure if you mean best RS skill or best IRL skill I gained from playing. I'll just add an IRL skill also: fast typing for sure as said below me.
The best skill to have gained for me has been the ability to keep cool when children with missing chromosomes try to piss me off in-game. XD
As for actual in-game skills; the best would probably have been Slayer. Fun imo & a source of easy GP.