Hey there! I'm a long-time Chomper main since GW1, he's my favorite character in the series period, and I've got hundreds of hours logged as him. So let me see if I can help.
1. Upgrades are totally dependent on what you want to accomplish. Do you want to get kills by swallowing players or by biting/breathing on them? If you want to bite or breathe, take the two upgrades that buff damage or breath duration and the speed upgrade as your third.
Chompula, Hot Rod, and Disco get a buff when they swallow zombies. Therefore my upgrade choices for the first two are two digestion upgrades (chew speed) and movement speed to circle around enemies faster, increasing chances of back-chomping them. This will ensure maximum chances of finishing eating to get the buffs. I don't have Disco chomper yet, but I'd take two digestion and perhaps forgo speed in favor of super meter.
For my spray Chompers, I take the overheat upgrade (not the penalty upgrade. Makes it too tempting to "floor it" rather than play well and think ahead.), the speed upgrade, and whatever upgrade will buff that chomper's spray, for example, the Fire chomper has a fire upgrade that's usually worthwhile. In general, you want to spray down zombies, not eat them.
The Yeti Chomper works best with freeze, speed, and damage upgrades in my experience.
The Armor Chomper MUST take the speed upgrade, but feel free to experiment after that, I use two chew upgrades.
2. Try using the super meter upgrade. Once again the RNG hath decided I shall not play this variant, so it's up to you to experiment and find out what works best.
3. In general, the stock goop is the best one despite the pointless nerf. The stock burrow is also typically better unless you're going to camp the garden on defense in which case the speed burrow might help grab unaware players. The chomp cannon is a good ability for the yeti chomper or spray Chompers if you want to stack ranged damage or spit it into the graveyard/garden to splash damage the zombie players into pieces. But, contrary to its description, it's not good for taking out roof campers in its current state. Super sticky (geen) goop is useful on spray Chompers if you want to make sure they can't avoid any of your spray, and the hot rod chomper since it's harder to chain swallowing zombies without getting hit and the slower cooldown works better with a hit and run playstyle.
The spikeweeds are a total matter of preference. I prefer stock.
4. I'm going to write a chomper guide this week it seems. Want to get the community's opinion. Here's some basic stuff:
- Save one spikeweed. If a zombie uses a skill that protects them from being eaten, like deadbeard's cannon, leave burrow next to them and drop the spikeweed under them. It'll yank them off the item and you can eat them. Wait for the item to disappear or you'll still grab it.
- If possible, get up on high ground, and wait for enemies to pass below. Goop them while falling and land behind them for a quick snack.
- Chomper is best when defending. So when you're on offense, please help your rose set up her magic teleporter base by eating anyone who would rudely interupt her building, usually an all star or super brainz.
- Soldiers can kill you by activating their ZPG just before your grab them via burrow. Wait for the rocket to leave the tube a while BEFORE clicking to chomp or you'll get hit. Usually a soldier trying to do this doesn't have a rocket jump left, they usually use that first.
- if an all star looks at a wall or his own barriers, don't chomp him from below until you know he's used his imp bomb. He can kick it at the wall to kill you in the same manner the soldier can.
- if you have to eat someone in a group, eat the scientist. He's the most likely guy to kill you at short range and he'd revive anyone else nearby.
- burrowing ahead of time and waiting for a while is a good way to eat inattentive engineers setting up the teleporter.
- if a zombie won't expose his back after they're covered in goop, drop a spikeweed at their feet, same as above.
5. Chompula, Hot Rod, Fire, and Armor are all great, Power is good too if the situation calls for it. The other Chompers are somewhat harder but the challenge can be fun.
Hope this helps!