Unquestionably you need to pick a plug made out of silicon, metal or glass. Cheaper materials can break down with long-term exposure to lube, or any exposure to lube good enough to keep you comfortable long term (like oil or silicone-based lube or Vaseline). I’ve been searching for the perfect long-term plug, and while I’ve come close I’ve never found perfection.
Larger toys are better (as long as you can comfortably take it) because they can be easier to hold in, are harder to “forget about”, and their bulk makes them more likely to stimulate your prostate which just feels good.

Small neck is important so you can relax your sphincter and actually get reasonably comfortable. Also a large difference in the max diameter of the plug at the widest point vs the diameter of the neck helps retention. A 2″ plug with a 1 & 3/4″ neck will just slip right out-a 2″ plug with a 3/4″ neck is much more likely to stay in place. The idea is to stretch yourself almost as far as you possibly can with the bulb of the plug, then let your anus contract and wrap tightly around the small neck. That way you actually need to make an effort to stretch again in order for it to come out.

While you are considering relative diameters, don’t forget about the base. You don’t want a toy with a base small enough that the entire plug can easily slide inside you. You could end up having a really bad time trying to get it back out, possibly tear yourself up a bit, and even maybe end up taking a really embarASSing trip to the E.R.

A comfortable base is the hardest one I think. Inside your ass, you’re much less sensitive than in your crack. You could be bleeding from torn tissue in your rectum & not feel it, but a tiny bit of chafing from the base of your plug can make your crack unbearably uncomfortable.
It doesn’t take much to make your crack hurt. I’ve vetoed many otherwise fantastic plugs because the producer just had to stamp their logo into the base of the plug 🙂
Some bases will feel great when you are standing, sitting, walking, or squatting, but none will feel good in all of those situations.

Some suggestions:

Square Peg Egg Plug (https://www.squarepegtoys.com/shop/egg-plug/)
Pros: Overall the most comfortable plug out there, very high quality. It is so comfortable that you’ll be able to take a significantly larger size than your other “largest” plugs (especially when made in their”SuperSoft” material),
Cons: A bit too comfortable, plus the neck:bulb size ratio is better than average, but not enough to make it easy to hold the plug in.

World’s Most Comfortable Buttplug [“WMCBP”] (https://www.mr-s-leather.com/world-s-most-comfortab…
Pros: Best neck:bulb ratio available. Neck is always about 1/4″ (heavy rubber tubing). Bulb comes in numerous sizes from “tiny” to “ridiculously huge”. Once inside you can’t forget about it because of the sheer weight of it, but it’s incredibly comfortable.
Cons: Rubber tubing being the only thing keeping a huge shaft of solid steel from getting stuck in my ass is not optimal, especially considering that if you are stretching your stretching limits, you’re probably using “oil or silicone-based lube or Vaseline” which the manufacturer specifically says NOT to do. To be fair though, given the weight & shape of the bulb it would likely come out quite naturally if it did slip in all the way. The major con is the base which can start to chafe really badly, especially when sitting.

NJoy PurePlug 2.0 (http://www.njoytoys.com/products/pureplug20.php)
Pros: Very comfortable, and great neck:bulb ratio. Best plug for prostate stimulation, & the uniquely-shaped base makes it easy to grasp for a bit of toy play, and in most situations the base is almost unnoticeable.
Cons: The weight and streamlined shape can make it tend to slip out, especially when your sphincter relaxes.