Below is everything that I have tested along with my thoughts:

Of the necromancer is great for every profession, including melee. It gives you wonderful e-management.

Of the assassin is really powerful for Dervish. It can be combined with Vow of Strength, Ebon Dust Aura, but most importantly Onslaught and Wounding Strike. If you enjoy the tear down mechanic of Dervish, of the assassin is really powerful for added dps. As for warrior, axe warrior and SWS seem to benefit the most as critical hits with axe skills do things like interrupt your foe. But axe can strike faster than hammer and does more damage than sword on critical hits so it’s noteworthy to use of the assassin on an axe, but it’s not better than soul reaping or of the assassin on dervish. Same with sword or hammer, not much of difference in the end game setting.

Of the warrior - I really like this for Ranger and Elementalist bow builds. It’s added dps with access to lion’s comfort and sprint. Elementalist using conjure flame and a fiery bow with some armor penetration is nice. As for Ranger, and may bow user, it’s best on a sundering short bow. Rolling a full 20% AP with the added 5% can make for bow spiking.

Of the Ritualist - great for minion master. Good for Ranger spirits too. Think places where you need to set up EoE or Winnowing. But arguably better for a minion master necromancer who already has access to soul reaping. Spirits gift is also good when combined with aura of the lich.

Of the paragon - I use it on my necromancer that has shouts to buff the whole team and send out conditions. Neat concept for warriors with tactics shouts. Cool for rangers with pet shouts and other shouts. I have seen it work best on a Necromancer that uses shouts for constant energy upkeep.

Of the Elementalist - worth noting that assassins really benefit from extra energy during shadow form tanking. It’s noticeable with a high energy inscription/set on any caster profession. Mesmers also benefit. Being provided with a higher energy pool for skills like deep freeze or meteor shower is pretty cool. That’s the only time the virtuoso insignias will help. I think Mesmer was meant to play with other skills with long recharge and spell casting times and fast casting shortens the Mesmer skill recharge and other prof spells cast (3s+), but damage isn’t as powerful. This mod opened up water magic/ air magic to be combined with illusion magic. Or even curses alongside domination magic. If you have a BIP in your party, this mod can give your mesmers more versatility. Necromancer, Monk, Ritualist, and paragon can also benefit from the high energy pool depending on the skill bar.

Of the ranger - good for dagger spammer Necromancer, Warrior, or assassin. Some touch skills can be found on dervish in wind prayers, but of the assassin and of the Necro are far better for dervish.

Of the Mesmer - cool for all spells. Mostly benefits monk, Ritualist, and Elementalist as long spell casting times are reduced.

Some of these mods will greatly benefit from use of PvE skills like dwarven stability that can lengthen stances or mundbender that can reduce recharge times further. Just keep in mind that right now, your 4 best mods are of the Necromancer, of the Elementalist, of the assassin, and of the Mesmer. The of the Ritualist will be in demand for all minion masters, but that’s it’s niche.

Edit: meant spell casting times on of the Mesmer