I wouldn't really say one pet is the best, because it does depend in part on what you fight. You'll get the best results looking at the strengths and weaknesses of your pet and setting them against an opponent where they will do the most damage and take the least damage themselves. Frenzies are probably the most popular pack though, so if packs are your interest then I'd get some. Ideally get them in all different colours then it's easy to know visually where the little gits are.
Some locations are just plain lethal to a pack even if they could theoretically do the most damage, so it's often worth scouting out in advance.But yes, smaller/pack pets in groups will do more damage than the greater dragon. Especially if you have another tamer with a tank pet or you add say bard skills in to support your pets further so you avoid the death domino effect where one dies and then everyone does. I find little pets survive best with my disco/provo tamer because she can weaken the bigger opponents and crowd control with provo. The provo is also handy to ensure a bigger creature is attacking another bit of spawn instead of your pets. But discordance is probably the small pet's best friend if you had to choose just one.
The steed and imp combo probably isn't as strong as it once was, I'd sooner have a pair of kitsune given they have well-rounded resists and can cure themselves etc. The steeds were great, but they couldn't cast cure so any magic or poisonous opponents could be extra tricky. I'm hoping steeds are one of the pets the devs improve.
With cu vs greater, the greater does more damage while it works as a tank, while the cu does take a long time to kill stuff. But the cu is only 4 slots so you aren't limited to being on foot, and you can also break up fights with a stop command, hop on the cu and run for it
In some areas having to run on foot with the GD still vulnerable to attack behind you... means the cu can be more useful in a tough spot
And my cu has saved my butt more than a few times with his healing. So I think they still have a place in the stables.
Wenchy