Which tank would you use?

It’s called “tank” because that is the position that takes the most hits at the beginning. If you look at the way the gems line up the tank takes hits from 3 of 7 columns. The other 4 positions get hits from 1 column each. So you want your most durable hero in the tank position, the one that can take the most hits and allow the other heroes time to charge their specials.