Can someone tell me what is needed in a good competitive pokemon team?

Right now i have:
Snorlax @ Leftovers
Careful nature/ likes to eat : Evs (140 hp 116 def, 252 sp def)
Curse : Rest : Body Slam : Earthquake

Electivire @ Choice Scarf
Adamant : Evs (252 speed/attack 6 hp)
Thunder punch : Ice Punch : Cross Chop : Earthquake

Umbreon @ Leftovers
Careful : Evs (128 def 128 Sp def 252 HP)
Curse : Baton Pass : Moonlight : Payback

Porygon-Z (when it evolves) @ ?
Timid : Evs (252 spd/252 spatk/6 hp)
Tri Attack : Nasty Plot : Ice Beam : ?

Heracross @ Choice Scarf
Adamant/ likes to thrash about
Mega Horn : Pursuit : Close Combat : Stone Edge

I also have a timid espeon, i'd like to input some pokemon from the sinnoh region or would like to get some pokemon that can learn stealth rock/spikes or replace some of my current pokemon's moves with stealth rock/spikes.
Any hardcore battlers that can help me?
What is on a traditional pokemon competitive team? 2 Sp Atk sweepers, 2 tanks, 2 Atk sweepers?
Help is greatly appreciated.

legendary's are always competive

yes but many people hate legendaries!

I hate hate when I battle people with legendaries on Battle Revolution. In my opinion you should ask your opponent if it's ok for them to use legendaries.