Randuin's Omen +500 HP +70 ARMOR Passive: Cold Steel Active Slow
Frozen Heart +90 ARMOR +400 MANA +20% CDR Passive: 20% AS Reduction on hit
Warmog +1000 HP Passive: Gain 1.5% Max HP in HP Regen
Runic Bulwark +400 HP +20 ARMOR +30 MR Passive: 10 ARMOR, 30 MR, 10 HP Regen
Banshee's Veil +400 HP +300 MANA +45 MR Passive: Spell shield every 25 secs.
Mercury Treads +25 MR 35% Tenacity
Would give you... +2.3K HP +190 ARMOR +130 MR 1.5% Max HP Regen and 10 HP Regen Spell shield, Extreme slow of attackers AS when attacked(Both Randuin & FH), and Tenacity
This would be the best general ultimate tank items. A few other come into play if you are talking champion specific, since people like Singed gain more HP off of MANA. As well as some champions already have passive ARMOR/MR from skills do getting items that give HP give more effective health. Ex. Cho'gath gets HP from his ultimate so ARMOR/MR give more effective HP than stacking HP items on him.