The smallest, weakest, and most bug-like of all MM sprites. Only half a fey RHD, which I assume can and shall be replaced by a class level (guys, please don't start a 10-page discussion on whether this is RAW here).
The chassis is neat, with bonuses to wisdom, charisma, dexterity, and even constitution, and no penalties except to strength. SR 17 is a lot at low levels, and DR 5/cold iron isn't shabby either. A fly speed is always welcome and makes the low land speed not hurt so much. Dodge and Weapon Finesse for free are icing, though I should note that without a source of bonus damage the second isn't going to be doing much for you.
But wait, there's more! Grigs also get a ton of SLAs. Some aren't that useful (Disguise Self?), and some quite situational (Pyrotechnics, Ventriloquism) but others are awesome (Entangle, Invisibility). In addition, grigs can have fiddles that project 30 ft. zones of save-or-lose, and although it's non-friendly being tiny and flying helps a lot with targeting.
I'm conflicted here: both +2 and +3 are appealing LAs. In the end, I think I will go with +3, if only because their fiddles are awesome (grigs without are probably worth +2). Either way, they can still get a lot of sneak attack damage or high spell levels, which I'm fine with.
A full hit die this time, and a size increase to small. Nixies can swim and breathe both air and water, but are incapable of flight. Their ability scores are okay, with especially high charisma. They, too, get dodge and weapon finesse as bonus feats, have SR, and have damage reduction.
Rather than the wide array of SLAs a grig gets, nixies must make do with an extended-duration Charm Person 3/day (CL 4) and Water Breathing 1/day (CL 12). They also have the empowered wild empathy most fey seem to get.
+1 or +2? I'd really like some input here, but for now I'll play it safe and go with +2.
Strongest of the MM sprites, and also most popular. A good fly speed, sweet ability scores, high DR, scaling SR, and the default sprite bonus feats make for a very appealing package, especially when combined with the phletora of useful SLAs and the nonmagical save-or-lose effects they get. Oh, and permanent Greater Invisibility.
Even so, I think +3 works here (come at me, vigorous debate!), with +4 or +5 (let's go with +4, just for more debate) for the variant that gets Irresistible Dance.