The afk player will always survive the longest. /thread

The most would be speed and keeping your distance. and not charging in to enemy one by one.

Rook, but only if you know how to handle him
(focus on speed, hp, and hp regen)
my Rook is always upgraded with speed, many say it worthless yet i out-run many DGs or run fast enough to
avoid death, combine with my good HP i can fight 1vs2 for a while, while my team mates bashing the enemy towers
then i run or teleport to base, heal, and do it all over again

it is also good for attacking towers, attack, retreat, capture flag, (by that time hp regnerated) attack again...

the variety of different answers you see here all stress one point: your question is very vague.

did you mean which DG can absorb the most damage? that would probably be Rook (highest health pool) or Oak (highest armor value). 

did you mean which DG can escape a fight most easily? that would be either Sedna (runs fast, regens health quickly) or Unclean Beast (runs fast) or Erebus (has a teleport type move) or maybe Oak (has a temporary invulnerability shield). 

did you mean which DG can stay on the front lines the longest? that would be Queen of Thorns or Sedna who both have alot of health recovery abilities. Oak would be a distant 2nd here with his Divine Justice passive ability providing decent staying power. 

you will notice that Torchbearer and Regulus are particularly vulnerable so they require a bit of extra care in positioning or retreating. on the other hand both of these DG's are very effective ranged attackers and benefit from some of the built in safety of range. 

Experimentally? Regulus.

When playing against Sedna, I find her actually hardest to actually kill-- because of the combination of heals and her speed buff. When I look at my actual death rate, I honestly think I die least with Reg. He's a total squishy, so this really, really shouldn't be (and in fact, I assume it's likely an abberation)-- but I go for the range upgrades, snipe, and speed items. He's extremely good at not being hit. 

Now he is NOT the most durable demigod-- there's a lot of 'and then I run back to heal' until you get the Heart. 

The one with the better player controlling it.

All demigods give their team a winning edge, if you're looking for someone who is most likely going to be easiest for you to keep alive as a new player I'd probably say try out Oak with his shield ability.

As everyone else has said though, all DG's can be super survivable if you play cautiously, just don't chase the enemy demigods, and if you can do it, try to stick close to someone else on your team