I still want to argue that Methos would always be a bit doomed with the Ring (see: when Kronos turns up and he backslides alarmingly) but I really like the notion that he's actually dealt with lots of way more evil things and it just has zero effect on him.

And really, someone should be like Tom Bombadil! Methos is an excellent candidate. (Um, now I'm ... imagining Methos and Tom Bombadil getting together for drinks and reminiscing and singing silly songs? They would totally do this.)