What is the most effective creature that can be resurrected from the graveyard in MTG?
The common name for this type of deck is 'Reanimator.' So a google for 'Legacy Reanimator' will get you what's popular for specific cards in the current metagame and from up to date sets.  I say that not to try to answer 'just google it' but that lists of cards can go out of date fast, so they're not great stack exchange answers.  Also the knowledge that the deck name to search for is 'Reanimator' is important :)

To try to make it an answer about the strategy/archetype more than a list that will go out of date with each new set:

1) Creatures with Shroud or Hexproof.  May be obvious but a lot of new players don't realize Shroud only functions on the battlefield and only applies to spells that say 'target.'  Creatures with Shroud are excellent entomb + exhume choices.

2) Creatures with an enters the battlefield effect that was at least a decent return on your mana even if they do get doombladed.  e.g., Bogardan Hellkite, Terastodon

3) Creatures that shut down your opponent's answers.  Iona, Shield of Emeria is the obvious go-to, but depending on your meta-game there can be more specific options.

4) Creatures that shut down your opponent's deck period.  Stormtide Leviathan doesn't have shroud, but lots of aggroy decks are counting on burn spells if they need to get rid of early creatures.  Even the Rubin Zoo variants usually only have 4 paths/swords so you've got ok odds game 1 when they don't know to mull aggressively for it.

5) Meta game choices.  You know your buddy is playing Stax?  How's he feel about Symbiotic Wurm turn 2?  Since you only need 1-ofs, you can have plenty of meta choices main deck.