That's a tough question to answer, both companies put out such great kits .
My favorite Estes kit would have to be the Andromeda , but the Blue Bird Zero and Mars Lander are a close second.
For Centuri, I'd have to say the Laser X, but again the Thunder Roc and Quasar hold a place in my heart.
My favorite Estes kit is by far the "Big Daddy" love that rocket. I owned the pre-"E" engine class and now the current one. Sure there's nostalgia for a Nova Payloader, or the Hercules. Never owned a Centuri kit so can't comment on those.
This one is very hard. I've never owned a Centuri kit, so no favorite there.
The Estes kit I most want is the Nike Ajax. However, I've never built one so not sure. Of what I have built, the Mean Machine has been my favorite. I'm about to built a special one for Southern Thunder. It's Estes tubing, but will fly on 29mm HPR motors. The hope is it will survive an I200 flight.
Estes - Cherokee-D, my first D powered rocket, and my first lost rocket. I've since built 5 more, including 2 different upscales, of which I've lost all but the 2x HPR upscale. I'll build a Semroc Retro-Repro soon!
Centuri - The Laser-X! I especially like the Semroc SLS (built 2, lost 1).
Centuri: My first serious Centuri model was the 1/100 Little Joe-II in 1970 (First balsa-finned model I built, previous 4 were MPC kits with plastic fins).
But yeah, the 1/45 kit topped it, THAT's my favorite. I did not get a 1/45 kit until 1975, when the local hobby shop owner offered an opened kit to me for $5 (cost around $13 to $15 at the time). I flew it a lot. Never mind the wrinkles in the body wrap or lost little pieices here and there. Photo below was in 1989 (yeah, that heavy guy is me on the left, Wayne Hendricks at right). I had modified it for clustered engines, 24mm center and up to six 18mm outer. That particular flight, I had modified the base of the BPC ("capsule") for staging, an A3-4t engine firing to push the BPC/LES off and deploy a chute by rear ejection (used a mercury switch to fire the flashbulb ignitor). Worked nicely.
Of course, my favorite kit now is the "Esturi" 1/45 Little Joe-II kit.....
I got into rocketry the first time around '79/80 when I was a pre-teen. All I knew was Estes. My first being the Alpha III, and I suppose that would have been my favorite. I didn't build a lot of Estes kits that I recall, having gotten a "Designers Special" box of pieces/parts for Christmas one year. There were plenty of Estes kits in the 80's that I wanted but never got - $$ was tight in our family.
Now that I'm a BAR, my current favorite is probably my Mega Der Red Max, but that's MPR. I like my Big Bertha a lot though. I've been steadily building for about the last 8 months and I'm getting a pretty good sized LPR fleet of the Estes kits I wanted back in the day... With many more to go!
I guess it depends on how you want to describe "favorite". And how old you are!
In one sense, my favorite Estes kit was probably the WAC Corporal. I must have built ten of 'em. It was a great performer, looked cool, was reliable and cheap (important when you are a kid). It was also a (semi) scale model, which I liked.
My favorite Estes model to fly was the Scissor Wing Transport. I got one for Christmas when it first came out. Absolutely spectacular to fly. My family still remembers watching it circle down to a smooth landing on the "lakebed" (local school playground was all dirt, Kinda looked like Edwards if you squinted a bit and used your imagination...).
Of course we all fantasized about the Interceptor. I didn't even see one in person, built or kit, until about 1980.
We didn't really get much in the way of Century stuff in my neck of the woods, though the occasional catalog would find it's way into our grubby little mits. There was a sense of "forbidden fruit" about everything in there!
Estes BBZ. Had to 'mail order' it, was one of my first ever catalog purchases with a money order and everything! very early 80's..
Century Hornet. loved that pointy nosecone. And from the card, it just looked 'fast'! And that was the first rocket I ever papers the fins. I hated the way the balsa looked, so I thought that adding paper would make them smoother & sleeker. I had no idea others did this..