Alright, my two cents here.
Infernape: A+
There are no safe switch-ins to the LO mixed set, meaning you are going to often have to sac off something and bring in a check. This means that every time Ape gets a safe switch, it is going to contribute assuming you predict correctly. However, Infernape also shines with its versatility, as it can easily be a dedicated lead, a full physical wallbreaker, or a scarf revenge killer. This makes predicting Infernape's attack even harder, and switching in all the more difficult. I feel that all of these pros are easily enough to warrant Ape in A+.
Volcarona: A
Yeah, the thought of Volcarona in UU last generation may have seemed a bit ridiculous, but it's here now, and we have to deal with it. Needless to say, it is very difficult, because very few Pokemon can take any of Volcarona's hits at +1, barring dedicated special walls such as Blissey and AV Snorlax. Plus, given its moderate bulk, setting up a QD shouldn't be hard, assuming you don't try to set up on Aerodactyl. Plus, there are many variations on the classic Volcarona set, such as a Roost variant, a ChestoRest variant and a Sub + QD set. However, I do not feel this 'mon quite deserves the elite tier of A+ rank, due to its counters (rock types, scarf Mirnshao), its serious 4MSS (can't run coverage and recovery) as well as its 4x rock weakness. Nevertheless, Volcarona is one of the biggest threats in the meta, and is definitely deserving of rank A.
Alakazam: A+
Wowie, what a wallbreaker. And a fast one at that- base 120 outruns non-Scarf Mienshao, Infernape and Galvantula just to name a few threats. However, as said before, Alakazam has potent offensive power to back up its speed, possessing powerful Psychic STAB off a very high base 135 SAtk, which is enough to put a serious dent in anything that isn't a pink blob. Combine that with perfect unresisted coverage in Shadow Ball, Focus Blast and Dazzling Gleam, and you have one of the best Pokes in the tier. And, just as the icing on the cake, its ability Magic Guard makes it immune to things such as hazards and Poison damage. And while its nonexistent bulk and one-dimensionalness keeps it out of S rank, anything lower than A+ is a crime against humanity.
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Mega Alakazam: S
I really don't feel like restating what others have said already, so I'll keep it short and concise. Honestly, Mega Alakazam is, bar none, the most threatening offensive Pokemon in the tier right now. Why you may ask? Well, look no farther than
base 175 SAtk and
base 150 SPD. This means that Mega-Sam is hitting the highest speed tier in UU, only speed tying with Aerodactyl. Plus, that SAtk stat means that this thing can leave massive dents in even the sturdiest of walls. However, the main reason that this thing is so scary, is its new ability, Trace. The amount of beneficial abilities that can be traced is RIDICULOUS. Want examples? Bring Mega-Zam in on a safe switch against a Nido. Trace Sheer Force. Yeah, now they're in trouble. Or, maybe you come in on P-Z and snag Adaptability. Yeah, enjoy taking Psychic now. Honestly, I could not find a reason to put Mega-Zam anywhere other than with the best of the best in S Rank.