Summoning unique monsters

[Ravingdork]

If you could summon one unique creature*, of a CR comparable to that on existing summon monster lists, what would it be?

I've found a rule that let's you do just that, but I want a good choice that is going to remain useful through a large segment of my character's career.

*:

That is, one not normally found on the Summon Monster lists.

[Coffee Demon]

What's the rule?

[mourge40k]

Adventurers.

[Ravingdork]

Coffee Demon wrote:

What's the rule?

From Inner Sea Magic, page 20 (rules text in smurf blood):

A student of the Acadamae may spend 3PP in order to obtain the Summoning Specialization ability.

Summoning Specialization (3 PP): Add a new creature to the list of options available to any summon monster spell you know. The new creature must be of a comparable CR to the other creatures available for that spell to summon, and you must have GM approval for the creature you add to your list.

[CBDunkerson]

Note: If you have the prestige points, there is nothing preventing you from taking Summoning Specialization (and/or Favorite Summons) multiple times.

As to what creature(s)... depends entirely on what the character (and party) needs.

[Avoron]

Summoning young dark shadows with Summon Monster III is pretty tempting off of the top of my head.

[Coffee Demon]

I like Doppelgangers.

[fictionfan]

Pugwampi their Unluck Aura is just as annoying at level 20 as level 1 which is to say really really annoying. If you cast spells with saving throws it's a force multiplier. Also they are CR 1/2 and so have comparable CR to the summon monster 1 list.

[parsimony]

There was a 3.5 option somewhere, where you only learn a new summoning creature type at each caster level. Also there was an option to roll 3d6 for the creature's stats (and roll its hit points) and the same creature appeared every time you summoned that type of creature. So the summoning process became part of a permanent relationship, although you could change out the creature on level changes.
I think the main thing about summoning is that the caster should be prepared with the stats so it doesn't slow down the game. I think this kind of thing is a great house rule.