Electrocute- Feels a bit clunky to get use to, however IMO it's only good in the mid or late game for burst openings compared to summon aerie which can pack good punch early and enough to not slack off late game. On top of with how electrocute works now it's "Weirder" to proc now for a few champs like those who could "2-hit" proc it, not necessarily harder. Better designed but at best it's value comes mid game+ which is great. I'd rate it 10/10 and it is the direction I wanted to suggest, less accessible early game but for a more powerful burst mid-late game. 4th most fun keystone for burst, idk about overall.
Predator- Free ghost+shiv is amazing for certain junglers, for laning it could see some play like for example hard engages to replace righteous glory maybe but it's main use for jungle is solid if chosen. I'd say it's about 3rd most fun for burst, idk about overall. One concern I did not test is whether or not it breaks stealth while channeling, just in case it were to could it be changed not to.(I'll try to remember to test when PBE comes back)
Dark Harvest- I Sadly did not get a chance to use it yet and I don't think "soul conversion to damage" numbers were shown to entice me into doing so. Can someone tell me the numbers for it if they were revealed?
Arcane Comet- I honestly don't find it "fun" at all, Summon aerie IMO is far better early game and doesn't remotely slack off late game, and electrocute is far better than it across all stages of the game. Sure you can continue using spells but having to hope that it lands doesn't feel reliable at all, unless you have some 5-10 second AoE hard CC spell to get value out of it like lux. The idea behind arcane comet is cool and it is less screen clutter than I thought it would be but I think it should have ONE of the following: Larger AoE, Lower Cooldown, Reliability by lowering the animation time or slightly more damage.
If not, it might be better off just replacing it with either electrocute or a different keystone, because aerie does more damage with more reliability and electrocute does better damage the whole game.
Phase Rush- Idk how I feel about ranged not getting partial slow resistance, but I'll wait until it's actually released because stormraiders and phase rush are powerful in their own right when people find them users who desire them. I love them, however one thing I will note, I did prefer having to reduce the enemy's HP by 30% for certain champs who had only 1-2 heavy hitting ranged attacks and it being changed to be like electrocute I don't know how I feel about that. For example Morde usually 1 or 2 hits reliably triggers stormraiders but now he has to always AA->Q->E, and stormraiders was used from W+E to actually land Q. Jayce did use it after a full range Q, but I agree he did not deserve having that. Some Nasus just one hit with Q and boom sticking power. One champ that heavily benefits from this is cassiopeia since any combination of spells and boom chase/kite power perfect for her. But I think I'd like to see the community's list of champs they use Phase Rush on vs Champs they use to run stormraiders on but can't really use phase rush for anymore. On a side note, Phase Rush/Stormraiders use to be my favorite but now it's 2nd overall most fun favorite and urgot's usage is pretty funny.
Summon Aerie- Last but not least my most fun and favorite overall keystone, not to mention it's simply adorable, easy on the eyes and way more versatile to proc than it's competition. Feels far more fun than the tad hard to get use to electrocute(which is better designed than TLD by a mile) or the completely redundant feeling arcane comet unless you think you can consistently hit multiple people with it as a damage dealer like lux double snare or maybe vel'koz but electrocute still better for them too. Summon Aerie delivers that early game damage even for melee units who in certain cases can proc it off multiple times in a duel or chase due to lower reset window since they are closer. Late game it may not deal as much as electrocute, but in a large amount of scenarios you'll be proc'ing it far more often which means if you're comfortable on certain assassin's kill window you'll still have bonus damage for the rest of the fight. My only concern is can decoys also have a fake aerie like how they were given buffs the real champ also got? Because I'm not 100% sure but if aerie is not on the clone it makes it far too easy to discern between the real and fake like when buffs and items were not visible on clones.
TL;DR- By far burst damage-wise (excluding Aftershock since I don't usually take tank keystones on assassins), I feel Summon Aerie is the best thing to ever happen to league and it doesn't feel OP at all, Phase Rush directly needs live testing but so far feels good benefit to new users vs cost of who loses stormraiders, Predator feels extremely solid for the jungle while lane could have it see some light, Electrocute is a solid mid-late game keystone and how it procs is far better than before's too easily to proc version, Dark Harvest I have to test when I get back on PBE, and Arcane Comet feels like it's outclassed by 2 other keystones and even if by itself doesn't feel "fun" to me.
Would love to hear other's thoughts on them, especially on Arcane Comet.