Best tank in 40K: Wave Serpent, hands down
Best IG tank: tough one:
overall - Demolisher with big gun, lascannon and 2 plasma cannons
anti-infantry - Hellhound (so cheeeeep), but the loads of dice from an Exterminator is fun too!
for sheer power-trip - Baneblade! (we had a tank company vs tank company battle the other day, including a Baneblade on each side - too funny, they are not as tough as they look)
"Cheesiest" tank: Landraider Crusader?
(ok ok, let's not go there - the SM players might just bring up the thing about wraithlords, etc.)