So this may be a little top-heavy, but I feel most gods are pretty good in assault. Mostly it comes down to how you play them, of course, but few are just poorly suited to the mode. I have Ah Puch, Hercules, and Zeus listed in S+ because they can dominate games by themselves. Ah Puch has great sustain and incredible burst. His weakness (lack of mobility) is mostly a non-factor in this mode since there's no jungle to gank him from. Hercules, when built around health and regen, is incredibly hard to kill. Zeus has ridiculous damage output with his bouncing chain lightning. Like Ah Puch, his weakness doesn't really apply here. In S tier I have Chaac and Guan Yu for their fantastic sustain. Chaac can heal himself immensely with one activation of his 3, while also creating large areas of slow. He can also output significant poke, especially if you have other front-liners and he can build power/pen. Raijin is here because he's basically a better Ah Puch from a damage perspective, and he has great protections for a mage, but he lacks the sustain of Ah Puch and the ridiculous constant poke to the entire enemy team that Zeus brings. S- has the gods that just bring a little more than average. Ah Muzen Cab has fantatsic sustain, a great ult, and gets stronger as his hives improve without having to buy items to gain attack & movement speed. Cupid can lay down a good amount of damage and cc and provide decent heals while he's at it. Athena can make sure the enemies' beads are always down, setting up all sorts of shenanigans with her taunt. With a good build, Bellona is extremely hard to kill and her ult seems like it's always available. The Rat has great sustain and can have 2 full items before his first death, since he can buy his acorn outside the fountain. Scylla's root and crush are extremely strong right now, and her ult can completely turn a game around. Chronos and Sol have sustain, intense poke, and can melt structures. Khepri has the cc and resurrection ult, plus the handy glowy ball thing. The A ranges are most gods, they're good but nothing really special. The A+ ones are gods that bring a little more to the table, like Agni's mana-less ult, Mercury's hyper-poke, etc. Hel and Ra are in A+ even though they have the best heals (possibly barring Guan) they're just too easy to kill. And they WILL be focused heavily. Hades is in a similar boat, because while he has an escape and a silence/fear, he also needs teammates that don't clear every damn wave before he can use the minions to heal you. A- has Bakasura and Zhong Kui because, while they can wreck face when played well, they require teammates that let some minions through. Down in B tier are the assassins that really need a jungle to be effective. One ranking that may surprise is how low I have Sylvanus. While his mana flower and aoe heals are great, he's just in a precarious spot. He wants to be the tank, but he's also going to be your only healer and therefore the primary target. He doesn't have the self-peel or movement ability to keep himself alive, and sacrificing power completely for health & prots weakens his heal.
EDIT: Added Erlang Shen in S-. I build full tank and just harass. His Pin ends up doing a lot of damage, in addition to the root when you land it, and his ult is a ridiculously good heal. I've rolled him once in assault so far and did just under 20k dmg in less than 13m with tank boots as my only power item.