Rally the Ancestors, Return to the Ranks and Immortal Servitude

All of the titled cards have potential in Modern but without a decent study on the format, I can't provide you with more evidence than my intuition. 

I picked Rally the Ancestors.

Reasons:

1. Instant itself is much better than Sorcery. We can cast it at the end of opponent's turn and responding to spells like graveyard exiling and shuffling cards. 

2. With 5 mana, it covers each 3 cmc or less creature, which is a range wider than creatures with 2 cmc for Immortal Servitude and three creatures with 2 or less cmc for Return to the Ranks. 

3. The "exiling at the beginning of your upkeep" side effect doesn't matter in combo decks. We probably want to use it in a deck built around a two - creature combo. The deck works when we get the combo pieces into grave via all sort of milling, entombing cards, then we cast Rally to bring them back to the battlefield and they 'll win the game for us instantly with an one turn killing effect.