I'd recommend going for ash first before putting more resources into your blast bomb. Or a cutter / dread venom. Or a gun (ele alchemer might fit into your combinations to deal with nasty zombies / robots / puppies). There are more occasions to take it with you than for graviton / nitro. Also the dot leaves you some room to experiment with other equipment while it's doing some of the work for you.

- Graviton is one of the best bombs IF you're in the right situation. That situation doesn't come to frequently in most lvls though and to create it, ash can be very helpful. I take it with me if someone manages to drag me to JK (no other way to squish out that much fun of those jellies)... and sometimes if I have a spare weapon slot and the environment isn't too dangerous.
- Nitronome can be helpful or lifesaving in many situations, but to use it effectively without killing & annoying your teammates too much, you AND your teammates need quite a bit of experience with bombs, so I'd recommend you put that somewhere further down on your list, too. I sometime take it to places where I'd usually take shivermist but the team is already strong enough to make it (faster+better) without ice, so I go for making room and some damage instead.
- Big Angry is a lot of fun, but rarely useful at all (compared to other things you can put into that weapon slot). Unless all of your teammates enjoy flinging around mobs, you might not be able to make use of it at all. The name fits it very well: If you're angry at a certain kind of mob it's the right choice. If you want to play effectively, it's not. I only take it with me to give those lumbers, trojans etc that seem to think they can just throw me across half the room every time I get a lag-spike a portion of their own medicine... also I like giving gremlin menders flying lessons, they're practically begging for it *grrrr*.