6 of a kind

Machamp, hands down. With DP (probably the best move in the current meta), he's more than useful in many situations. VS Blissey, Snorlax, T-tar, Lapras...

This is my current personal goal, I already have 5 lvl 30+ and looking for the 6th one with great IVs.

I started out with 6 Machamps, then 6 Golems (rock), 6 Tyranitaurs and 6 Rhydons (ground). I guess all them are very good in their respective niche and will keep being that in the future.

Any that are the optimal pick for something you really care about accomplishing. Ranking them is hard because most of their worth is subjective. 6 Machamp is optimal for shortmanning Tyranitar. 6 Exeggutor is optimal for soloing Omastar. 6 Raikou will be optimal for maximizing damage balls against Kyogre. That said, one mon in particular stands out:

Tyranitar. There are numerous likely future raids where it will be top counter and its high stat total and lack of relevant challengers in its types make a 6 Tyranitar team likely to be a long-lasting investment with significant returns over the years.

EDIT: Attack mons I have teams of 6 or more: Tyranitar, Machamp, Golem, Dragonite, Rhydon, Exeggutor, Vaporeon, Jolteon, Flareon, Espeon, Gengar. Used to also have 6 Cloyster for the old gym meta.

Tyranitar with Bite/Crunch. Tyranitar with Bite/Stone Edge. Machamp with Counter/Dynamic Punch. Golem with Rock Throw/Stone Edge (although that might change with Gen3). Exeggutor with Confusion or Extrasensory or Bullet Seed/Solar Beam.

Champ
TTar
Raikou

In that order.

Golem is too big of a risk
Dragonite is too general
Mewtwo is obvious but too rare.

Exeggutor would probably be 4th

I'm trying to have the more Pokemon with different movesets as possible. Here are mine:

Nidoking and Crobat - I think thay can be good in the future against Fairy
Alakazam
Machamp
Golem
Kingler - I believe he will get a good moveset in the future
Exeguttor
Rhydon
Pinsir
Gyarados
Eevolutions (Vaporeon, Jolteon, Flareon 'n Espeon)
Snorlax
Dragonite
Scizor
Heracross
Donphan
Tyranitar - For me, one of the most important. I'm keeping 10 and will search for 14 or 16 (6 B/C and 6 B/Se, the rest are for possible moveset changes)

Somebody somewhere caught their 6th 100 Suicune, all with different powers...but this is not a joking board. I did catch a 100 electric today...might max it to 2000 soon;)