Chaos Daemons (Mainly Tzeentch, I like the screamer model, the good psychic phase, the invulnerable saves and option of deep striking)
Tyranids (Flying hive tyrants are a nice model, but I don't want to spam them)
Genestelear cults (Very expensive right now, but seem fun to play and have nice models)
Imperial Guard (Nice tanks, lots of people. Hammerhead gunships will be rather annoying.)
Bearing in mind that I haven't played these armies, I play Mechanicus, I would say:
Chaos Daemons - Fallen is right that these guys have the biggest learning curve in the game. Start off playing the game with them and you WILL lose a lot at first, and while the army-wide invulnerable saves sound good on paper when you actually roll ten dice for a Bloodletter squad under fire, you'll see quickly how easy it is for your guys to be wiped out. And all it takes is a single bad roll on the Warp Storm table to turn a battle you were winning into a rout. A good Daemons player can run roughshot over other factions in my opinion, but it takes time and practice to get there.
Tyranids - These guys might be a good choice, but from what I understand at the moment they are not very competitive due to a lot of their upgrades being useless and currently having the most worthless units in a competitive sense in a single codex. Flyrants armed with Twin-Linked Devourers are the unit you likely refer to there, and I have heard they are baller powerful. I can't really say anymore on them since i've never played against Nids or seen anyone who does play them at my local GW. That said Nids have the best starting set in the game with the Tyranid Swarm boxset (https://www.games-workshop.com/en-GB...uestid=5732399
) which will give you a good solid core of Gaunts and Gants with some Gargoyles to support them and the tarpit to end all tarpits, Ripper Swarm models.
Genestealers - These guys have some damn interesting deployment rules if you go for that stuff, and they have a cool amount of variation on them thanks to the looted Guard aspect and the Cult Troops aspect, you can really create a personal force with these guys. To their rules I don't know a great deal, but they can get a good psychic phase with the Magus and Patriarch while having a lot of rending troops and cheap Neophyte fodder.
Imperial Guard - Their tanks are deceptively strong, seriously the armour value on a Leman Russ is insane. Even shooting at it with a AP1 S10 weapon isn't a guarantee of hurting it! But the Guard require you to make a choice early on, at least at the points you've chosen, and stick with them. Do you want infantry or tanks? Do you want a Mechanized force (re, transports) or an army of footsloggers? Do you want battle tanks or artillery? Mix and matching with Guard doesn't seem to work from what i've seen, you need to focus on one aspect early on and as the points climb higher then you can look at adding different wings of force into your army. That said at 850pts troops will likely have to be your choice since the good IG tanks aren't that cheap.
One thing I would say though is do not be discouraged by any advice here. Even if you pick a hard to play army like Daemons or one that is currently underpowered like Tyranids, don't let the fact that you'll probably lose a good deal while learning to play them in comparison to a more beginner-friendly army like the Tau or Space Marines. If you want to play Daemons, play them. Even when losing you'll still have a good time. When I started playing Stormcast Eternals in AoS, which are pretty beginner-friendly army to learn though very hard to master, I still got my ass stomped half a dozen times before I pulled my first win, but even the losses were still fun because none of them were shutouts, each time I learned more and still made some good gains until I finally understood the strengths and weaknesses of the units I had. Every army takes time to learn.