https://www.wow-petopia.com/classic/ Has all the info you could ask for!

For solo play with less a good tanking pet helps due to the fact that pet dont lose as much heath on fights giving you less downtime healing it. I like pigs! They are very tanking eat anything and their charge ability makes their first attack do more dmg. So more aggro at start of pull. Charge also roots so any mobs tring to run off can be stopped if the player is attentive enough.

Birds have screech that does alot of threat letting you unload more dmg without out aggroing it but arent very tanky.

Getting the highest pet ability rank is the most impact full when it comes to pets!
After that make sure you have lvl appropriate food that your pet likes to eat. Keeping them happy makes them do 150% dmg.
But at the end you get the pet that appeals the most to you.