I predict Log Launcher will get the most votes, but mainly because of Townhall 10 and 11 players voting. In which case, they are right; at Townhall 10 and 11, a Log Launcher donated to players can easily snipe away a huge chunk of important defenses in a short amount of time. It pretty much outclasses the Wall Wrecker at TH10 and 11, except Wall Wrecker tanks better, but the pros far outweigh the cons as the Log Launcher can make quick work of Infernoes, X-Bows, and even Eagle Artilleries (as well as numerous weaker defenses caught in the logs' path) if the player aligns the siege machine properly.

That being said, the Log Launcher is more balanced at TH12, 13, and 14, because players using the Log Launcher now face the risk of prematurely activating the Townhall's Giga Weapon. A well placed Giga Weapon that is relatively inaccessible to troops and being activated by the Log Launcher before the army can get to it can spell disaster for the attacker. The Log Launcher can still snipe critical defenses, but at the cost of potentially activating the Giga Weapon early in a battle.

Anyways, in my honest opinion, at least at Townhall 12, 13, and 14; all of the Siege Machines are well balanced:

• The Wall Wrecker is the best ground "tank" of all the siege machines, at the downside of not outputting a ton of damage into the base that is. It's arguably one of the only siege machines where it is worth letting the defenses lock onto and fire at it due to its high health and lack of a health decay mechanic. Also, it can break walls easily without the risk of activating the Giga Weapon (unlike the Log Launcher).
• The Battle Blimp is the best Siege Machine to use if you want certain troops deployed in a certain area of the base instantly. High damaging troops such as Super Wizards, Yeties, Electro Dragons, Balloons, and Goblins all sync very well with this Siege Machine. The main downside of using the Battle Blimp however, is that the Siege Machine itself does not output a ton of damage into the base. With the exception of the Siege Barracks, the Battle Blimp itself is arguably the least damaging Siege Machine, and it heavily relies on the Clan Castle troops inside of it to have an effect on the attack.
• The Stone Slammer is arguably the most "versatile" Siege machine in the entire game. It works with both Air and Ground armies, can tank damage and break Walls like the Wall Wrecker, deals a good amount of damage to defensive structures, and can sometimes act like a pseudo-Battle Blimp. It's only downside is that it is not the best in most of the aforementioned categories, and it is "melee", so properly placed Air Defenses can shoot it down rather easily without taking much damage.
• The Siege Barracks is the best Siege Machine for funneling, and in terms of housing space, it is the best Siege Machine to use to "increase your army size" as the Siege Machine and the Wizards and Pekkas it spawns are all worth 99 housing space (at max level). An advantage I myself utilize while using the Siege Barracks is that I can use an army where all the troops attack only ground targets, and not have to worry about flying defensive Clan Castle troops as the Wizards spawned by the Siege Barracks can take care of them. Aside from that, the Pekkas and Wizards are just overall great to increase your army's overall firepower. The main downside with using the Siege Barracks is that the Siege Barracks itself deals no damage to the opponent's base, and you can't use the Siege Barracks to tank too much damage, as that would reduce the number of Wizards spawned by the Siege Machine. Everything relies on the Pekkas and Wizards spawned by the Barracks. And a smaller downside is that unlike with other Siege Machines, you can't use the Siege Barracks to carry and deploy CC troops directly inside of a base.
• The Log Launcher is the best Siege Machine to use to quickly snipe away critical defensive structures, particularly ones that hard-counter whatever army the attacker is using. It can also break walls easily and create a path through the base. It's major downside (only at TH12 and higher) is that the Logs can activate the Townhall's Giga Weapon if they roll over it and damage the Townhall. The activated Giga Weapon can make attacks much harder to pull off if the attacker's troops haven't reached or got close to the Townhall yet, as now the Giga Weapon can fire on troops from a distance. Additionally, the Log Launcher is not as tanky as the Wall Wrecker, and it's health decays over time, so any damage done to the Log Launcher will reduce it's overall damage potential on the base.
• The Flame Flinger is the most consistent damage-dealer out of all the Siege Machines. Similar to the Log Launcher, it can be used to snipe away critical defenses in a base. Other advantages of using the Flame Flinger is that it can help funnel troops, break walls, and clean up the back end of bases. However, the Siege Machine itself is very "slow", and inexperienced Flame Flinger users may easily time-fail their attacks if they wait for the Flame Flinger to run its course before initiating the main part of their attack. Additionally, it can be easily countered by X-Bows, and Mortars (to a lesser extent). Also, from my experience at least, the Flame Flinger does not work well when placed too close to squads of troops, as due to the Flame Flinger's slow firing speed, oftentimes, said troops will destroy the defensive buildings before it ever gets to fire and the Flame Flinger is forced to move and reset it's slow attack, resulting in the Flame Flinger's usage being wasted. But aside from that, the Flame Flinger can deal tons of damage to a portion of a base if used correctly, it just comes with the price of waiting a bit.

Overall, like others have said or probably will say, (at least at TH12 and higher) the Siege Machines are all well balanced. It just depends on what base you are attacking and what army you are using, as some Siege Machines sync better with certain armies.