Six features of your aider carabiner
The carabiners on the business end of your aid ladders will probably become your single most used piece of hardware. Choose them carefully. When you find a good one, buy two, because you need one for each aider. Note these are all my personal preferences; consider it a starting point. Try a lot of different styles in your training and see what you like.
Consider these qualities for your aider carabiner:
Non-locking - Sounds obvious, but I’ve seen beginners have a locking carabiner on their aiders. This is a carabiner you’re going to be opening and closing hundreds of times a day on a big wall, and you for sure don’t want a locking mechanism getting in the way.
Oval (or other standard shape) - Depending on how you set up your aiders, you might be clipping several things into the bottom of the carabiner. Having the classic oval design gives room at the top to clip a big wad of webbing, and at the bottom to get in whatever else you need. Having said that, some people prefer a slightly easier handling of a bent gate carabiner. Generally, avoid a very asymmetrical design.
Standard size - You're after the Goldilocks carabiner, not too big, not too small, just right. You don’t want a mini carabiner; harder to clip bigger random things. And, no need for a wide mouth belay style HMS carabiner either.
Solid gate, not wiregate. Wiregate carabiners are generally hard on your fingers to open hundreds of times a day, solid gates are a little more finger friendly.
Thin nose and thin metal stock - Having a narrow profile, or “nose” at the top of the open gate makes it easier to wiggle the carabiner into occasional small loops of webbing, rivet hangers, etc, and to unclip under load.
Distinct color - You can either buy an odd colored anodized carabiner, or even spray paint something creative yourself. Why is a distinct color important? You want to be able to glance your gear harness and always grab the aider fast. (Yes, you should get into the habit of always clipping your aider into the same spot on your harness every time, but having a distinctively colored carabiner makes it even easier to locate.)
Note that these carabiners are going to get pretty beat up, especially if you are clipping lots of bolt ladders. Check it regularly for any small burrs of metal that might hook up on webbing, and retire them if you need to.
The Black Diamond keylock oval carabiner, a tried and true design, and the Petzl Spirit, are both solid choices.
non-locker
classic oval style and bent gate with lots of room at both ends
medium size, not too small
solid (not wire) gate
thin / narrow nose
okay, not a distinct color for that oval, but five out of six ain't bad. All you need is some spray paint!
(I think BD also makes a non-keylock oval carabiner; be sure you get the keylock, aka notchless, version.)
Also, check out the DMM PerfectO, a classic shaped oval carabiner that comes in lots of cool colors. No spray paint needed!
image: https://www.backcountry.com/dmm-perfecto-straight-gate-carabiner