interwyrm said:
(adjustments are based on going from a standard kobold)
Green Dragon gives +3 natural armor, +6 str -2 dex +4 con
Copper Dragon gives +3 natural armor, +4 str -2 dex +4 con and Tiny Size
Pseudodragon gives +3 natural armor, +2 dex, +2 con and Tiny Size
There are probably better options for alter self as a humanoid. All three of those seem at least worthwhile to me though.
Alter Self doesn't change your ability scores; you get natural armor, size, natural weapons, and movement modes (within limits), racial skill bonuses, and racial feats. So if you turn into a Pseudodragon, you gain three additional points of natural armor and one additional point of size bonus to AC, Fly 60(Good), and Weapon Finess (as well as a bite and sting attack).
A Wyrmling Black adds +1 size AC, +2 Natural Armor AC, move 60, fly 100 (average), swim 60, and three natural attacks (bite, claw, claw)
A wyrmling green adds +3 natural armor, fly 100 (average), swim/move 40, bite/claw/claw
A wyrmling white adds +1 size to AC, +1 natural armor, move 60, swim 60, burrow 30, fly 120 (average) (capped by Alter Self), bite/claw/claw.
A wyrmling brass adds +1 size to AC, +2 natural armor, move 60, burrow 30, fly 120 (average) (capped by alter self), bite/claw/claw.
A wyrmling copper adds +1 size, +3 natural, move 40 fly 100(average), bite/claw/claw
White for movement, copper for AC, Pseudodragon for manuverability. Oh, and dragons are natural spellcasters, so under most DM's, will be able to cast spells.
Note: AC modifiers listed as relative to the non-spelled kobold