Boy, this is a hard question. I currently have five, and I love them all, though one of them is a 1/16 customized nitro that isn't of much use other then goofing off.
I started out with a TMaxx in 2008, and it was my re-introduction to RC after about 10 years of inactivity. I probably bought every hop up part you could before it even saw the dirt, lol. I joined a local club and there were lots of IS MT track racing in those days, so I really had a great opportunity to race and learn from people. It was my first nitro machine, and in those days everyone was running nitro. I had the TRX2.5 engine, which I promptly tore apart and started tinkering with. My truck was pretty heavy and didn't have a lot of power, but it handled great, and had nice balance for jumping with soft landings. I didn't bash much, just raced on tracks and stuff, so it took some abuse, but aside from the wheelie bar and bumpers, nothing much ever bent on it.
This was the truck in 2008:
I had so much fun with that little truck! Then it sat for a few years after I got divorced and remarried, and collected dust. My second wife would ask me if I was ever going to run it again, she said the little truck looked sad just sitting there, so I dusted it off, got it running, and did some more upgrades.
This is the truck now:
It's main changes were the new shoes, and powerplant(Dynamite .19). I changed the stance a bit for more suspension travel. I still love driving this truck at high speeds around tracks, it is so much faster than it was. For pure driving enjoyment, it's really my favorite. But, I have to say there are things about TMaxx's I dislike, especially how difficult it is replacing some basic parts where you need to rip the entire suspension apart, but fortunately I've not needed that on this particular truck(my other TMaxx using mostly stock parts is another story). I also despise the pillow ball style steering knuckles. It's just a terrible design, IMO.
I knew at some point I was going to start building and racing solid axle trucks, but I was unsettled on the platform. I really wanted to do nitro(and I contemplated converting SMT10's, Ground Pounders, etc). I initially was lured to XTM X-Factor trucks, but being no longer in production, and lack of hop up parts was a turn off(later I would realize that you don't need much in the way of "official" hop ups for solid axle trucks). There's a lot to like about those machines, and I'll still probably get one some day. Eventually though, the Mad Crusher came out, and I really liked the look of that chassis, especially the 5-link suspension. IMO the truck still has some design issues, like the horrible steering bracket that clamps onto the axle(it's weak, flexible, and just why??). So I snagged one, and right away, much like in 2008 started replacing and reconfiguring things before it ever saw the dirt. I found working on this truck vastly easier than the TMaxx, and while there was some trial and error with customization, the fact that I could do so many custom changes was a huge plus for me. I suppose that is true of most solid axle trucks.
My Dragonoid Fire truck is a blast to drive, even around a track, though it's really meant for quickness and smaller jumps ala Monster Jam or Penda style racing. It's a bit overpowered, (was) a bit finicky and kept breaking stuff, but over time it's become pretty durable and reliable. Just for the pure customization level, engineering, and that it's easy and fun to work on, I have to say my Dragonoid Fire is my favorite all time truck (for now).