Here is the maximum possible health regen/5
Dr. Mundo with 4 full Warmogs, Spirit Visage, Force of Nature, an active Elixir of Fortitude, and and active health potion. All health and health regen masteries, Quints and seals of Endurance, marks and glyphs of Vitality.
Lulu with 5 full Archangels Staffs, Rabadon's Deathcap, active Elixir of Brilliance, Ignite on cooldown, Surge active, all AP and mana masteries as well as Summoner's Wrath, Whimsy cast on herself, and a full page of knowlege runes.
Sona with Hymn of Valor stance active.
Taric with Radiance active.
Heimerdinger just hanging around.
Between Sona Taric and Heimer, there must be a Locket of the Iron Solari, an Emblem of Valor, and a Will of the Ancients.
Mundo and Lulu must have Baron buff.
Lulu uses her ult on Mundo, then he uses his.
This succesfully gives Dr. Mundo 10650 health and the highest possible health regen/5 at 4660.606.