There is no such thing as the strongest. You can train any of the neopets up to as high a strength as you can. Plus, all the neopets can use the strongest weapons as well, so there is no difference between them except for looks.
Some pets are stronger than others in some areas when you create them. For example, the Acara will start out with onlly a little bit of HP. On the contrary, the Bori will have a lot more HP than the Acara when it is created. There is no way to tell which pet is the strongest on neopets though
To make your pet strong though you can go train them at the academy or the ninja training grounds.
Academy - requires dubloons
Training Grounds - requires codestones
The higher the lvl of your pet, the more dubloons/codestones you need. They are pretty expensive. Some also use the lab ray, but you need to collect all 9 pieces of the map first. This is even more expensive and is random. It can also change your pet's species and colors. It's price is a little under 1mil neopoints. yes, 1,000,000 neopoints. It is convenient though. It goes at about 900k.
If you have anymore questions, neomail me at peach123131
There is probably a neopet that is strongest, but we can't tell. Super-high level neopets have there stats marked as "too high to count"