I'm not sure you have a clear understanding of the term "tank".
A tank is something that is built to take a lot of damage and dish out a lot of damage (like Mega Venusaur).
I think you are thinking of a wall, who's basically meant to tank hits and cripple/stall out the attacker (like Ferrothorn). These are the ones who usually have 252 HP / 252 Def / 4 SDef and vice versa.
Okay, now that we're done that, here is my advie for walls.
Physical Wall: 252 HP / 252 Def / 4 SDef
Special Wall: 252 HP / 252 SDef / 4 Def
Mixed Wall: 252 HP / 232 Def / 24 SDef, or something of that nature.
Whatever you do, do not split the EVs into 252 HP / 128 Def / 128 SDef or 252 Def / 252 SDef / 4 HP. This leaves your wall unable to take general hits since you either have weaker-than-optimal defences or not enough HP. Usually a mixed wall leans towards a certain side of the spectrum, while leaving a little bit on the other side. For example:
Ferrothorn – 252 HP / 212 SDef / 48 Def
This lets Ferro switch into Special attacks really well, but he can also take Physical hits better.
Here is a quick overview of a tank.
Special Tank: 252 SpA / 56 HP / 200 SDef
Physical Tank: 252 Atk / 12 HP / 244 Def
Mixed Tank: 252 Atk/ 252 SDef / 4 HP or 252 SpA / 252 Def / 4 HP
Note that these EVs are not set in stone. You can alter your tank/wall's EVs to suit your particular needs. Want to EV your Mega Venusaur to suvive Talonflame's Flare Blitz? Maybe move some EVs from Sp. Attack to Defence. Is your Latias taking too much damage from Salamence's Dragon Pulse? A few EVs in HP should do the trick. Be sure to customize your wall/tank to suit the needs of your team.
