I'm going to run through every Mega Evolution capable pokemon and list where I think they belong without their Mega.
Venusaur - RU
Before it's Mega, this things claim to fame was it's Chlorophyll ability. It's no longer as prominent with the weather wars being nerfed. It's typing with the addition of fairies is now much more beneficial.
Charizard - BL3
This thing does have a wide variety in the move pool, it does a lot of things good, but nothing great. The 4x weakness to stealth rocks REALLY hurts it's chances.
Blastoise - RU
This thing is still a pretty solid defensive spinner. It's got decent HP and two solid defenses both over 100. Not to mention water is only weak to grass and electric, two easy types to support with team building. With access to Scald, it could and Rapid Spin, it still can be useful in the ranks of RU if you ask me.
Alakazam - BL
Honestly, I think this thing was a bit too good for UU with it's focus sash, magic guard, speed, and coverage. It might not be the greatest in OU, but I think BL is just right.
Gengar - OU
Very versatile, still used in OU considerably. Great defensive typing, immune to 3 types.
Kangaskhan - NU
This thing has some interesting parts to it. Early Bird + Rest can allow it to wake up after only 1 full turn of being asleep. Run it bulky, throw in a sucker punch for priority and it's got a nice little niche. Good stat distribution, biggest issue is that it is fairly lacking in coverage and easily played around.
Pinsir - NU
I mean... literally this thing is outclassed by Heracross in just about every way, who is now UU. It only gets one useful stab move, X-Scizzor, which is meh.
Gyarados - OU
Very good, very versatile pokemon, can be offensive or defensive. Stall breaker, set up sweeper, you name it.
Aerodactyl - NU
It's got good speed...... I guess. It's fragile and has many type weaknesses. Not much room to set up to sweep and no useful abilities.
Mewtwo - Ubers
Next....
Ampharos - BL3
It can be a bulky pokemon with above average special attack, Electric only has one weakness. But it lacks coverage.
Scizor - OU
I don't think I need to explain this one.
Heracross - BL
I think UU for this thing is kinda underrating it. It's very flexible in it's abilities with great coverage. Not the fastest mon in the world, but it can be a great wall breaker.
Houndoom - RU
Right where it's at, seems like it fits.
Tyranitar - BL
It's been falling lately while Hippowdon has been on the rise. It's not a great defensive typing, and kind of slow. But it still has good attack and natural bulk. As well as good coverage.
Blaziken - Ubers
No... just no.
Gardevoir - RU
This thing doesn't get enough credit if you ask me. It might not like steel types, but you won't find too many in RU. It has a good typing, but it doesn't pack a huge punch and requires a turn or two to set up and be a serious threat in RU. It also has Will-O-Wisp
Mawile - NU
Absolutely pitiful stats, but good moves, good typing and two good abilities to choose from.
Aggron - RU
Dislike the typing, it's defense is still off the charts, and it has good attack, but it's not going to survive much on the special end.
Medicham - RU
Hey.... it's got Pure Power... a couple of priority moves, and not a bad type combo.
Manectric - NU
Nothing really special about it. It's incredibly similar to Zebstrika without beating the 110 speed tier, though it does hit harder in the special side.
Banette - PU
There's nothing much to like about Banette. It's Prankster ability helps it greatly, but otherwise...... I just can't find anything to do with this thing.
Absol - RU
It has good attack, Knock Off/Pursuit/Sucker Punch are always good stab moves and it also had access to Swords Dance and some good coverage, but it's frail and not very fast. Sword's Dance + Sucker Punch are going to be it's claim to fame.
Garchomp - OU
Great mon, nuff said.
Lucario - UU
Great movepool and it's versatile. ESpeed it always nice. Abilities are ok, but this thing can do some damage.
Abomasnow - RU
I'm getting tired of writing things here so I'm just going to give the rankings now.
Beedrill - PU
Pidgeot - PU
Slowbro - OU
Steelix - NU
Sceptile - NU
Swampert - BL2
Sableye - BL
Sharpedo - RU
Camerupt - BL4
Altaria - NU
Glalie - BL4
Salamence - BL
Metagross - BL2
Latios - OU
Latias - OU
Rayquaza - Ubers
Lopunny - NU
Gallade - RU
Audino - PU
Diancie - UU