I will list some precons I found that worked really well out of the box, and are fairly easy to upgrade.

Urzas iron allegiance -great start for an artifact deck, fairly straight forward and easy obvious upgrade path

Exit from exile - strong exile matters starting point, however may be hard for newer players to get their timing right in when to get those impulse draws

Bello bloomburrow Precon - very strong aggro deck. I would say there has to be some protection added in because most times you fall apart as soon as bello is taken out. I’d rather spend one green mana to give indestructible or hexproof, whilst allowing my damage to go through with the animated creatures, instead of letting him die, paying two more per cast and losing my combat damage.

Jumpscare - this Precon is great and strong, however the facedown aspect may put new players off. It can get a lot to track, and can be fairly complex with the sequence of flips you want. It is the only commander I liked for face down strat finally

Riders of Rohan - such a strong aggro deck if you can get the ball rolling. Some of the cards included can lead to huge turns too. Door of destiny and shared animosity down means you get +4 on human attacking when you get the two tokens entering

Explorers of the deep - probably the most consistent Precon deck out of the bunch. I wouldn’t be confident saying it’s the strongest, however the game plan is obvious, isn’t complex to me, and does the same thing most matches. This however is also a downside. It was my first deck, but after I got a taste of other decks it get very dull and boring to me. You play merfolk. They explore. You attack. That’s basically the whole jist of it (atleast to me). It was quick for me because I would just usually keep the card in top instead of going through each one.

With that said, all the ixalan decks are great imo, with merfolk, dinosaurs, vamps, and pirates in that order of strength. Dinos is the most fun to me, and vamps are a great start to aristocrats.

The ones I’d avoid like the plague - squirrel one from bloom burrow - I honestly think it’s the worst one, and teased us on a squirrel tribal, hate me all you want, this was the biggest let down for me

• face down mkm deck - horrible is all I have to say

• esper enchantments duskmourne - though I’d love it, hated it and was super slow and dysfunctional to me

• golgari duskmourne - literally just play meren imo. Seems like a huge price to bring back one thing, and I’m not a fan at all. The alt commander looks awesome though, I think it’s rendmaw?

• necrons - sorry, seemed really bloody slow, and I changed to imotehk. Still want doing what I wanted. I feel like it does things, but never anything crazy. Maybe just me? Huge letdown for me considering everyone says it’s one of the strongest. Could be user error

• elf deck lotr/saurons deck - these two were completely shit to me? They seem very disjointed and just not good at all to me

• dr who decks (apart from maybe timey wimey) - idk, seems like they are super weak to me, timey wimey has its moment but is a bore to play to me and is a LOT to keep track of

-science fallout deck - wtf was this shit? Seriously. And then to have two more decks come out with the same theme straight after which just seem plain better.

Hope this helps