The new MM does not have a stat block for a monkey much like a Spider monkey. I was working on a character that has a monkey much like the one you see entertainers have that sit on their shoulder. I was reaching out to you the community in hopes of making a good balanced stat block for it.
Thank you for any help and or assistance.
probably
str 10, dex 16, con 10, int 6, wis 6, cha 12 (cause monkeys are adorable)
Treat tail like a light mage hand, weight limit 5 lbs or so. advantage on climbing checks with pro. in acrobatics.
I'll pipe in here. As a zookeeper, I will tell you that a spider monkey would require one hell of a shoulder to sit on 
Think Capuchin, like the monkey you see with the organ grinder, or as Indiana Jones' sidekick. This is an animal with no real strength or HP, but very useful as a distraction, i.e. pulling hair and biting ears. They can be quite a terror. In fact if they were really feeling it, they could rip your face and ears all to hell.
Also consider that monkey are actually trained to commit burglary and pickpocketry in real life, so skills like slight of hand and stealth are good ones to include. In the way of stats, I'd look at them like a 1HP creature.
Just my 2 cents.
Also, I'm speaking as an animal person here, not an experienced DM.
The Capuchin was what I was thinking up. The entertainer monkeys that work with the guy with the music box or the one from the Jack Sparrow movie that was named jack. Thinking that the size would be Tiny since they are about the size of cats. Low str , higher dex .
Thanks for the submissions looking forward to some more.
I would use the stats for the cat in the back of the PHB. Simular size and weight. That is what I did.
Daniel
My monkey familiar. I'd say the climb speed and Standing Leap ability are most important; otherwise with a Str of 2 it'll fail all of it's Athletics checks for climbing and jumping!
Monkey
Tiny beast, unaligned
Armor Class 13
Hit Points 1 (1d4 − 1)
Speed 20 ft., climb 20 ft.
STR DEX CON INT WIS CHA
3 (−4) 16 (+3) 8 (−1) 4 (−3) 12 (+1) 6 (−2)
Skills Acrobatics +5 Sleight of Hand +3
Senses passive Perception 11
Languages —
Challenge 0 (10 XP)
Standing Leap. The monkey’s long jump is up to 10 feet and its
high jump is up to 5 feet, with or without a running start.
Actions
Bite. Melee Weapon Attack: +0 to hit, reach 5 ft., one target.
Hit: 1 piercing damage.
Mechanically it's a mish-mash of the Ape/Baboon (mental stats, climb speed), Frog (Standing Leap), Lizard/Spider (size appropriate climb speed), and Weasel (physical stats, skill modifiers). I gave it no more than two skills, in line with other Tiny Beast familiars, and a +0 attack bonus (same as most familiars, Bat, Cat, etc) because it feels useful enough for utility that granting a modifier here seemed unnecessary.