Generally , besides phoenix and Aquadharma, the strongest pets are the ugliest,lol.
Before you can afford a phoenix, Baby Hercules, or Golden Turtle (or are lvl 99+ to farm them), I would recommend a Molten Lava Crystal which is pretty even on all stats.
But there are different pets for different things, for example:
Air:Phoenix- Best air, best pker(speed)
(If you can't afford Phoenix or are a low level)
pick a
petite sawflyGround:Aquadharma- best pker (for a ground pet)
Hercules- Best physical defense (8952 at lvl 90) and can be buff itself
Venin Scorpian- Best damage (for ground pet) : 3336 dmg at level 90
Erinceous Cerus- Best lure (can get farthest away from you without running back)
Infant Antilope- Best magic defense for ground (8662 at level 90)
Water:Golden Turtle- Not sure of the stats and never seen it but seems to be strongest so I hear but here are two other when you can't afford him.
First off: Water pets are barely used and so are hard to level. Most werefoxes who don't care about having the strongest water pet will simply catch a water pet around his/her level, abandon it once there is a huge gap between the new mob and pet, and capture a new one.
Besides that the
Abysmal Pond Turtle is the best besides the legendary.
And if your lazy but still want a decent water pet, get a
Celestial Ichthyoid , although they are level 80 so choose from then to cap random pets around your level or take the time to level up a turtle.
See some stats at
http://ecatomb.net/petstat.phpor simply google pet stats perfect world which will show other tables with same stats but without pictures so it's easier to compare instead of scrolling back and forth.