I was going to say 1 car, and just list 5 of them that I have on Media, but since people are doing top 10s, I guess I am too. By the way, this is the list for me that impacted to me positively in Forza Horizon 4. I have other cars I liked better for sure. I guess if you want, there's a link to the car in mention on the title.
Special mentions:
AMG Hammer: The real rubber shredding, insane Mercedes. Just deserves a mention because it exists.
Volkswagen ID R: Masterpiece, but I don't find myself using it for any real reason.
Ford GT '05: A more personal choice. It felt so rewarding to drive, but alas: it's meta.
Quadra Turbo R V-Tech: not the best crossover idea, but the result was a nice dirt car.
Lamborghini Urus Concept: I made it for S1 Road that somehow worked. But another car has a better claim for good chaos.
Lexus LFA: lives and breathes with the sound in mind.. which is why it didn't make it
Mosler MT900S: reminiscent of my first serious racing sim in GTR2. Also the must have car for free-roam. Nearly made the list, but one car overtook it for me.
10: LEGO Speed Champions Ferrari F40 Competizione:
I don't like S2. The experience is coarse, any mistake you make will be rough, other cars are irritating and gets everywhere. I probably have to mention you gotta pick fast or swift, and very few cars accomplish both in good level. Most Ferraris with aero will satisfy this well, and my choice in this case would be the LEGO F40. I can't find anyone making good use of this car, and outside of Winter, I can always rely on it to give me good S2 positions, always reaching the podium at all good times. Special mentions here would be the regular FXX, or even the base F40 Competizione.
(NSFW link) And since you know it's coming, let's get it out of the way.
9: Dodge Charger SRT Hellcat
There's bound to be some sort of muscle car in this list. So, recently I've been doing seasonals, one being a Modern Muscle in Winter. I so happened to pick up the Charger Hellcat for the job, and on ice: it done me so good, I've outran the AI fairly quickly. Better yet: it wasn't converted in any way, so I sort of have a new appreciation for the car even though it's tuned. I can choose this car for burning rubber, or racing comfortably. I've had other bigger muscle car love for its rivals, but I think the Charger shined for me when it mattered.
8: Aston Martin One-77
One of the first cars I got for the game back in March. It's not represented well in terms of performance, especially in most other games it's in. But it not only looked like genuine art, it got me the initial confidence to plow through higher levels of performance early in my game experience. Even before I started fine tuning the cars I owned to drive, this car was so nice and gentle to me for a luxurious, rear wheel spinning, monster V12 under a British racing marque.
7: Toyota Land Cruiser Arctic Trucks AT37
Right now, my family owns the regular variant of this exact Land Cruiser. It's on the list because of that. I call it the car that rides with the other Toyot-erm Baja WPs in S1 Cross Country. It's a total sleeper, and is a true test of my dexterity, especially in Winter. I think for me, this symbolized the ridiculous, chaotic aspect of Forza and other racers that allow this sort of thing to happen in this level. This car isn't basic, but seeing domestic everyday cars going 300 KM/H is just the sort of silliness and amazement we need sometimes in life. Let's not get started with the Peel P50/Trident in Online Adventures.
6: Ferrari 166MM Barchetta
Now, the game is full of old racecars. And when I mean old, they really mean it. I got bored and decided to go the auction house one time and got this thing for cheap. Guess nobody wanted to cruise in a red barchetta. Then came a little stint with me riding this thing in A Class winter. It didn't win, but got third place. On winter. It's not tuned to race for winter. That inspired me to not give up, and I started loving this lil' boat. History dictates that this car was the very one that put Ferrari on the map. It may sound like a dumb move, but that fact had me do something.. maybe a little ways mad. The 166MM was something I've been hoarding for a while now, with 6 in my garage, each serving just about any restriction within Online Adventure (bar S2, one of them is stock). A seventh is coming for drifting. My favorite one is for A-class road (linked above), mainly because it's the first, and built just as pure as the stock one. Didn't put high enough on the list because it's more a fun set of cars than for serious use. Also:
I needed to put Red Barchetta somewhere, so.. eh.
5: Toyota MR2 AW11 Supercharged
I have to pick a JDM car sometime, and how about another Toyota, but a normal one? No, not a JZA80 Supra that's being meme'd to death in this game. Here's the one I went for: my childhood hero the AW11. I've had a very limited experience driving this car in games, the last being.. an expy in Wreckfest. I sort of felt bad for not realizing it was in the PS3 era GT games. One of my favorite 80s cars because it was daring, experimental, and sort of nuts. I got in and boy, it did not disappoint what it can do. Find me in game, and you might find me with this cruising by the edges of the map.
4: MINI X-RAID All4 Racing Countryman
I don't quite like MINIs. I also already did this silly Star Wars meme, so I'll cut to the chase: incredibly stable with a bit of understeer. Championship winning. Tall like a SUV. For a MINI. It gets points for being so funny, but successful. I'm also a fan of the (not really) hard-to-find X-RAID Buggy too. My go to for a not so serious run of A-class Cross Country. While I treat it similar to the 166MM (I have 3), I place it higher because it works to my needs much better, and isn't too rare, and not Credit heavy.
3: Hot Wheels Ford Mustang
I had to choose a Mustang. It was going to be the '69 Boss 302, but it wasn't much fun to drive as it was taking photos of it (and bias to the 429 that wasn't in). It was going to be the RTR Spec 5, but I couldn't get it to be driven hard without rewinding time again and again. It was going to be the '20 Shelby GT500.. actually why wasn't it the Shelby GT500? I think because of this charm the Gen 5 Mustang had when it was revived. This charm wasn't in that new Mustang. So I picked a toy car. So what? It's crazy. It can do corners. It's fun. I might have to toss a coin if it weren't for how much I liked the Gen 5 Mustang better.
2: Porsche Carrera GT
My take on Porsche has been how I like making fun of the cars being so similar and boring. It does mean they're consistent in making them work, and work good. But sometimes, you take a dare, and come in with something out of line, but still work. They're good at that. And the Carrera GT had me that impression from when it was just a concept back in the day. Over the years, despite the definitely not a driver's car status, it still remained as my favorite super car, waning away slowly by other more sophisticated ones. Now, I know it as my favorite Porsche, still one of my top 10 supercars, and lives on as that, with also being best V10 boi (change my mind). In this game, maybe an off-meta? Keeping it the devoid of conversions for S1 road, I don't think so. At least I hope so. There isn't even a livery or custom paint job for this thing because I don't need to paint it to show off how good looking it is (except that one time in the original NFS Most Wanted, but let's leave that there).
1: Ford Lotus Cortina Mk1
I recalled watching that Grand Tour skit about this little Lotus. It never occurred to me a certain car old men liked would be so well loved, but I didn't really get the memo. Then I saw a video of it racing against other classics in Goodwood last year, and boy did I become a fan of such a quirky little car (as well as the Studebaker Lark Daytona, and the Ford Galaxie 500, but back to the Cortina). In the game, it's underrated and criminally underused. I have 2. One stock for my pleasure, while one is an all converted variant for running with those nasty meta cars in A-class. Performance wise, I can see it not being top level. But it's just the car I sort of wanted with a 400 HP package in barely a ton lugging about. Just behind them until they mess up, and.. the Cortina's gone. Plus: it works well in snowy conditions too. I plan to buy more of these, maybe one for S1 if I feel like messing about. This is probably one of the cars I really hope they didn't forget to import to Mexico later this year.