Is it old? It's only in it's 16 season, when you consider Corkscrew, an off-the-shelf, cheap ride designed for travel and small parks lasted 28 years, and could have lasted longer, Nemesis, an extremely compicated, expensive and brilliant ride designed to last, will be around for a lot longer than 30 years. It's barely reaching middle-age.
To add to that, a B&M rollercoaster has yet to be removed or "destroyed" from old age. So whilst they may say their rides have 30 year life spans, there is no reason at all that Nemesis can't go on and on. As long as Alton maintain it well, and lets be honest they have done a cracking job with it.
I'll say Rita. I know it does well with audiences and on score surveys. However to me, it just doesn't look that permanent to me. Whilst it would take a lot of effort to replace Air, Nemesis or Oblivion, Rita could easily go..
This is all conjecture though, realistically, the big 5 could go on for years and years.