I think ab roll outs are the best core exercise period, but you need to have a lot of core strength to do them properly and safely. Edit: but you should do hanging raises too, because they are also awesome.
Yeah, ab wheel is probably one of the very best, that's why a lot of strong guys are only doing this one together with heavy compound movements for training their core. An alternative to the ab wheel would be doing planks. There are all kind of planks. You can begin with the standard plank and make it more difficult while you progress, either by changing your stance or by adding weight or keeping the position for more time. This one is considered a very good variation you might want to try [ame="https://www.youtube.com/watch?v=6TKktamzq4o"]Updated RKC Plank - YouTube[/ame] Leg raises, sit ups, crunches and other traditional exercises are out of vogue right now, but since so many generations became strong and ripped using them they can't be that bad me thinks. This kind of training is more popular these days https://www.t-nation.com/training/21st-century-core-training https://www.t-nation.com/training/secret-to-ab-training
They're still really difficult to do properly. If you don't already have a lot of core strength, it's really difficult to maintain a neutral spine through the movement. I've watched a lot of people perform them with massive amount of lumbar extension, which is really bad for the spine and takes all of the load of the abs. All pain, no gain.
In those cases I'd recommend starting with very partial reps (limited ROM) and progressing from there.
lat strength is way more important than preexisting core strength when progressing to rollouts. the more weight you can hold on a fully flexed shoulder, the less you need to compensate with lumbar extension to roll out further.