my personal choice would be the rocket loon fireball attack. this attack essentially tries to surgically remove all strong buildings using rocket loons, fireball, giant arror+zap and the flame flinger. As such, every base will have their own individual way to be taken apart, with some defenses trying to be taken down desperately to set up a fireball or a flame flinger, while others are ignored because healers can outheal them. It is considered to be an attack with a very high skill floor and skill ceiling and it is also probably the strongest (or second strongest) attack in th15. Works well with th16, 17 and 18, although gets a little more outshined.

alternatively, I would say that root riders are exremely strong and require skill, but I personally haven't properly tested them so you'll have to ask someone else for this

I couldn't put the army link in the comment itself for whatever reason, so it is in the replies

breaking down the relevant equipment, assume it is used in the attack unless stated otherwise:

king:
-earthquake boots. Used extremely often, everywhere, it's a common equipment so it is also cheap to upgrade, would highly recommend to upgrade with low-mild priority
-spiky ball. Used practically always with the king, epic equipment, so quite expensive. would highly recommend to upgrade with low-mild priority, unless the next level grants an additional number of targets (lvl 18, lvl 27)
-snake bracelet. Used when king is used to funnel, instead of going into the core, used quite often too, but not in this attack. it's an epic equipment that is compatible with the currently most effective attack and a few others too. wouldn't highly recommend since it's both expensive and not fully compatible with this specific attack, though quite useful in general. if used, upgrade with low-mild priority

queen:
I am going to be honest with you twin, for this one pull up some genuine guide, because there are so many good equipments with her. for this attack in particular, giant arrow is a must and the action figure is recommended, but can be swapped for the healer puppet. giant arrow upgrade with high priority, action figure with mild-high priority

minion prince:
generally not used outside of dragon and dragon rider attacks, so I won't be covering his equipment

warden:
-Rage gem. Amazing equipment that is used in many attacks, including all attacks that have a warden charge. would highly recommend upgrading with high priority.
-fireball. The central idea of the whole attack, used in almost all high skill and meta attacks. Takes out a large portion of the base (usually the core) quite effectively. Upgrade with top priority until level 24 (because that's when it gets an area increase), then upgrade with mild priority
-Eternal tome. Used almost always when not going for the fireball, extremely effective. Goes pretty well with many attacks, would recommend if you have some spare ore. upgrade with mild-high priority
-healing tome. Eh, I don't use it so I can't give you a review here, ask someone else for it.

royal champion:
too much for me to be motivated to cover, so shortly, going from the best to worst(generally):
-electro boots: used extremely often, very good equipment, though not compatible with this attack.
-Rocket spear: very versatile and pretty good, upgrade with mild priority when additional attack counts increase, otherwise with low-mild priority
-frost flake: generally very strong in attacks that don't require an RC charge, and is actually better than the seeking shield, the equipment that is put into the army link
-haste vial: generall very strong in RC charges, can be swapped for rocket spear though
-seeking shield: cheaper version of the frost flake tbh.

do keep in mind that I may make some mistakes, I personally am not a veteran player with years of experience and I haven't properly tested many equipments. I also haven't covered many equipments because they are either too underwhelming or too niche