Good question.
I love lacies, but, and this is a big but. hahahahahaha they were my childhood fantasy. Kinda like wanting a supermodel. You dreamed about it, but never thought it would happen. And when it did happen, you found supermodels were a little on the dangerous side. hahahahahahahaha. Thats the case with lacies, they are great fun, but the males are a little dangerous.(favorite of large monitors, even thought they are not that large)
The gouldi complex, are far and away the most fun. Ranging from Sand monitors, V.flavirufus, to V.p.horni, on the extreme end. They do it all, and cover all behaviors. They burrow, climb, use water, no water, stand up, roll over, etc. hahahahahaha. They are sweet, they are monsters. They are active(not boring), and they are tolerant of a wide range of conditions.(favorite for med monitors)
Then ackies of all types. While they are not my favorite build for a monitor, I am not a fan of short stocky monitors. They are by far amoungst the most entertaining. Actually I like their builds all up until old age. They are colorful(can be) outgoing, and are the greatest of teachers. They will beat knowledge into our thick skulls. And they are like all other monitors.
Ackies are extremely curious, even in nature I found them with their heads stuck in crevices(and dead) in beer cans(and dead) at the bottom of ponds(and dead) They just cannot stop that curiosity. They do this in captivity as well. (favorite small monitors)(close call here thought)
I really do not understand keepers, they seem to be all about PROTECTION of their animals, without ever knowing their animals. I say this because these animals(ackies) love to investigate new things. They can be a never ending supply of entertainment. But its up to the keeper.
Try putting something new in the cage, then see what happens. Try hanging something new in the cage(just out of reach) and see what happens. Of course the general behavior is easy to predict, they will hide, but then they come out and try and figure out what the heck is in there. They work so hard at doing that its absolutely insane. If you hang stuff, you may see that they will even push things under it to climb up. Its hard to say they did that on purpose, but it always ends up with stuff under the object.
Why ackies are great is, they get over their fear in a big hurry. Most australian species are like this to some degree. While most indo species are on the other end of the scale. Their fear(survival mode) overrides their curiosity. There are exceptions, both in species and individuals. This is simply a general rule. African species are just flat dull to me. Only because they fit in the middle. They are behaviorally average.
But don't forget ALL MONITORS, are behaviorally expressive compared to many other reptiles(snakes)
ALso the gouldi complex are not prone to biting for self protection. They are also not armed with horrifying teeth, nor do they bite hard. They DO HAVE the ability to make you bleed, but not much more. I believe I can be considered an expert on this subject. hahahahahahahahahaha ARGUS are the worst of this group in this area.
Don't get me wrong, I am keen on Croc monitors and peachthroats. I also fell in love with one individual Bluetail. She was one of the sweetest monitors I ever kept. I picked her up for a friend and only kept her here for a month or two.
I also like the nub-tail group. Thats the true dwarfs, like V.caudolineatus, V.gilleni, V.kingorum and of course the the favorite being crosses between the first two, which I call caudolenis. I have had more fun with these then any other type of monitor ever. My guess here is, when you cross species, you eliminate or mute, species specific behaviors. You end up with a monitor thats all monitor, without the defensive adaptions that allowed them to survive and specialize in nature. This has been true with my gouldi complex crosses as well.
These crosses tend to teach you what MONITORS and their true behaviors are. Which I believe is not very well understood. BY ANYONE, muchless science. Sadly, most biologists(mechanics) and academics and thus authors, appear to not know their bum from a hole in the ground with it comes to behaviors.
Which leads to this, my favorites are always the most expressive and behavioral of the varanid species and individuals. AFter all, if you keep them for any lenght of time. WE the keepers have to be entertained. Cheers