This blog post is no longer updated, but providing possible tanks for classifications through comments is still accepted. Please refer to the new subpage here.
Ok, here is a blog post about different classifications of Diep.io tanks. Some may kinda be new on your ears, but it is because of what I thought on my mind. I encourage anyone who read this to say what tanks should be classified as (e.g. Boosters as Rammers), by providing it in comment section so that they can be easier for new users to use these tanks during game. A tank may be classified as more than once (e.g. Fighters as Rammers and Glass Cannons).
The Balanced Ones
These tanks usually focus with their stats being balanced (all 33 skill points distributed almost equally) to be poweful. (Very) few tanks are classified as this, and most users are usually newbie players, which should not be a problem when dealing with them. Be wary, however, if the user is skilled enough, you may get unbalanced yourself and die from their balanced prowess.
Tanks usually under this:
Tanks being classified as this are usually versatile in almost all condition, yet not so powerful. Compared to Balanced Ones, All-Rounders doesn't always use equally distributed stat points as their build, but mostly what they seems to be useful (and really happened to be). They can also sometimes go by themselves, even without aids of other tanks.
If you know what a battering ram is, then you should know what is this. Rammer tanks are typically fast, durable moving with their health and body damage usually maxed out for maximum destruction of their target. Their build also accompanies high health regen for faster ramming attempts and reload for recoil boost of their barrels. For some rare instances, once two rammers enemy to each other collide, both may die, but don't consider it useful when attempting to fight a rammer as a rammer.
The Glass Cannons
As what Zathus once said, a glass cannon is something with EXTREME OFFENSE but with USELESS DEFENSE. In other words, it uses high bullet firepower in exchange for its health-related stats. Difficulty when fighting these classification of tanks depends on what you and your enemy is using, but in general, getting the first successful hits is the key to beat your enemy, before the brittle firearm blasts you out.
Supporters, as what their name implies, are tanks whose purpose is to support someone during battles. These are very common in team-based game modes, but rarely in FFA-based game modes. The build they use depends on the tank they use. For instance, Trapper users focus on having their traps high on health and can damage heavily, and that they can replenish them fast with high reload. Supporters may rarely go for kills, but they usually do if they can (and they want). It is usually recommended to kill supporter tanks first so that other tanks have lesser chance of winning against you.
It may sounds weird due to the main entities he player controls are tanks, but tanks under this classifications are pretty strong and tough to fight off. Their build usually focus on having high offense and defense at the same time. Movement speed, if not upgraded, are one of their problem, but usually solved by means of their barrels' recoil. Better get prepared when you faced one of these, perhaps a grain of salt would help.
The Bullet Spammers
Tanks under this classification are considered as fans of "spray-and-pray" technique. Of course, their main key here is their high fire rate. In fact, they also use it to farm polygons for points. Although their bullets doesn't always do high damage, a bunch of these is what anyone being in a situation of fighting Bullet Spammers fears of. Consider being thrown with multiple pebbles, if that hurts, then being hit with multiple bullets hurts more. Fighting these tanks can be difficult.
Campers gonna camp, watchers gonna watch, and lurkers gonna lurk. These are the tanks preferred to stay in a position while killing their targets, without much problem dealing of against them. They are pretty tricky when choosing their positions which they feel comfortable, and do what a real sniper does. Of course, they typically had higher FoV than other tanks, and some even uses invisibility for maximum hiding technique. Beware when you are going, they might be watching, even near to your base (in team based game mode).
You may already have your mind what are the tanks in these classification. Automatics uses weapons that can attack on their own (auto turrets and drones), even without player intervention. However, player can still control them if they want to because of, well, important reasons (fighting enemies). Their builds typically uses high firepower. Difficulty when fighting them depends on what tank you and the enemy use.
These tanks are not able to be verified under any other classifications, due to their special properties. However, their special properties is what the game modes they are present use. Too overpowered, they cannot be upgraded to and played in normal circumtances. Some of them can only be defeated by team work, except for the one "too bounded from the power of legendary Green Triangle (if you know what I mean)."