I'm going to assume by your wording "strongest" is in terms of soloing,farming,pvp'ing and support.
There is no strongest class in AQW.
Every class in AQW has a purpose and an attribute that makes them shine.
When comparing SWOT to VHL, you'd see it's a multi target class with long range that excels in farming and dealing nukes,its dodge rates also makes it a good class in terms of pvp'ing,farming and soloing too so of course you'd think its better because it can multi target. Sure VHL it's a single target and doesnt have dodging capabilites, However it is a strong soloing class and has excellent endurance,deals quick and large burst damages with low cooldowns that can farm enemies faster than you think since it's designed specifically to kill enemies with large healthbars swiftly.
What SWOT lacks compared to VHL primarily is the availability for players, the ONLY way you can get it is by spending AC's, whilst VHL can be earned for absolutely free but be prepared for the terrible RNG and boring repetitive farming if you do not intend to spend money. (I spent 5k ac's buying Crag and Bamboozle just for this class).
SSOT can also run into mana problems if not used correctly and is quite unique compared to other calendar classes as it has different functions.
VHL however is just a spamming class that requires a no-brainer to use since it's user friendly.And you know what they say, the more complex it is,the more it's easier to break,so remember.
Complex class ≠ Better Class
It is easier to just buy SWOT but it is by no means a replacement for VHL.
What these classes both have in common is that there is absolutely no support skill's.
So you'd be thinking "If SWOT only costs' about 24.95$ and is almost toe to toe with VHL,I'd wonder what I can get for more?"
Even the most expensive class Master of Moglins is good in farming and soloing, but support skills depend on luck and is quite bad in pvp'ing
Great Thief bests in soloing and pvp'ing,but in terms farming it is abysmal as it can run into mana problems and lacks any AOE and range capabilites.<- and this means alot for classes worth more than 100$.
If AQW did have a class that bested in all categories such as farming,soloing,pvp'ing, and support.
All the other classes that existed before it would have no meaning as they are completely inferior compared to it,and it is also impossible to implement such a class because of the limited skill set.
So in conclusion,
I absolutely dont know what you want but if you mean by pure strength then go for calendar classes like SSOT/SWOT since they can deal nukes quite often to their opponents.
Void Highlord and SWOT when compared to each other are almost similiar.In my opinion,I'd think VHL is better than SWOT because simply of endurance and mana sustain.
These classes are not exactly the best in every category but they do what they can in what the norm of players would find in a class.
Edit: What infuriates me most is that alot of the classes in the game are very grindy or either super rare and strong which just fucks it all up for the new players.They should stop doing "rare" exclusive classes and just focus the "rare" label onto decorative items.This can atleast balance it all out for everything, but no, my name is not AE, its Mr Krabs' and I like money.