Goldfish, most probably. They are known for living upto 30 years. If u feel that they are old common, boring fish, try cichlids, they live over 10 years, are beautiful, tough, resistant to many diseases . Even catfishes do the same, most of them living for ten yrs. The best thing is go and do a thorough study yourself before taking anyones advice. Heres a good link, with very detailed info about many fish varieties. Outlook of the site is a bit outdated but it is one of the most vast fish & small pet index u can find on the web.
http://www.aqualandpetsplus.com/