Suicune or Lapras/Snorlax/machamp

Depends on what you need, really.

Of the ones you mention, Machamp is the most useful overall in the current meta, but if you already have a bunch of them then it is a different matter. Lapras isn't what it used to be and Snorlax is a dedicated defender in a meta where defense isn't terribly important.

I think just do raids for your conveinience. Don't 'go out of the way' to do a Suicune if there is a Ttar next to your doorstep. I would do Ttar Snorlax and Lapras by virtue that they are the only Tier 4s that people will bother with.

Suicune - why give up when you only get a 91iv.
Atm Vap is better, but What if suicune gets cp boost in the future?

Only for the rewards or if Niatic tweaks the CP formula again (which seems possible). It is silly that Suicine is outclassed by Vaporeon, and I would not be shocked if Niantic did something about this.