I don't know how MysticUmbreon makes his lists, but what I've seen he mostly takes account of type matchups, which is a big mistake cause movepool and stats also matter. Especially in Reborn.
For Reborn what you need to take account of is consistency. How consistently a pokemon can stay in your party through the game and pull its weight.
After a lot of very different playthroughs I played myself and other's people experience I have seen I can tell you the bellow pokemon are very consistent
1. Blaziken: Undeniably the best starter! There is not even a debate about it. There might be a few starters that have slightly stronger early game like Charmander with Dragon Rage, but Bulk Up via lvl up with Speed Boost it just breaks the game.
2. Magneton/Magnezone: I know that this line has been pushed post Agate now, but it is THE best Electric and a great Steel type. This combinations is just so good that it outshines any other option even at the point you get it. It's lvl up learnset is enough for what it need to do and it can take advantage a lot of field effects. I can argue that this one is a bit debatable cause it's just so late in the game, but in previous episodes it was in Shade's Gym and most runs I saw from back then 80% of those used one.
3. Blissey: This one is a hidden gem tbh. It is overlooked so much that it pains me that people don't understand how powerful of a choice it is. First of all it's a guaranteed encounter. You don't have to search for it and it is given for free for completing the Day Care side quest. You can get Chansey as soon as you can mine for Oval Stones, which is a high rate item to find, and you can get Blissey instantly if you bother to grind for friendship. But even without grinding friendship, Chansey is still good enough for what it needs to do. It may be fragile on the physical side but you are using it wrong if it has to face physical attacks. Also, as soon as you get access to Mawile in the underground railnet you can also breed for Seismic Toss which is infinitely your best way to deal damage outside of Julia's Charge Beam. Last, a great niche I've found is that Softboiled can save you so much money on healing items. You can actually use Softboiled in the overworld to heal your teammates in exchange of Blissey losing some HP, but then you can easily replenish it by initiating a wild battle and use Softboiled to heal herself back. I can understand why not many people don't use it cause it just slows down the battles a lot and Reborn is a very long game. But Blissey is so great that I totally suggest you to give it a try.
4. Mamoswine: Best Ground type easily. The combination of its STAB attacks is amazing and half of the weaknesses of the Ice typing is covered by Thick Fat and the Ground typing. You can get Mamoswine quite early right after Rock Smash if you want to bother breeding Ancientpower from Dunsparce to Swinub. But even if you don't want to breed, you get the move relearned after Aya so you can reteach it with the cost of a Heart Scale. But imo, you WANT to breed a Swinub cause unfortunately Mamoswine's lvl up learnset is a bit whack. 1st of all your best Ice STAB from level up is Ice Fang which is very weak. 2nd Ice Shard is only learned from Swinub and not from the other 2 evolutions, so if you somehow forget Ice Shard and then evolve it you can't relearn it (or catch a high lvl Swinub and evolve it). So the best thing to do is to breed a new Swinub to get Ancientpower and Icicle Crash (you get it from Alola Sandslash and Beartic which are also available at the same time) and you'll never hapen to miss Ice Shard while you lvl up your new Swinub.
5. Heracross: It might not be available that early but the timing it's perfect for it! Available right before the Psychic and Dark Gyms makes a great choice to face those leaders. STAB Close Combat and Megahorn are extremely powerful coming from 125 Attack and coverage moves via lvl up with Bullet Seed, Aerial Ace and Night Slash compliments it greatly. I do not remember if the Event Heracross can come with Rock Blast as an egg move, but if it does you should go for it! 3 great abilities that you can pick which one you want for each major battle also gives a versality you wouldn't expect.
6, Flygon: One of the earliest "good" Dragon types available with a great typing. Flygon with its Levitate ability can provide great resistances/immunities like to EdgeQuke coverage or to Electric/Fire coverage from things that lack HP Ice. Flygon's best point comes from its movepool. With Earthquake from lvl 33, with Superpower from Trapinch at lvl 40 (you should not evolve Trapinch as soon as possible, Vibrava is ass), with Rock Slide for great coverage and Dragon Claw as soon as you evolve and you get almost unresisting coverage. And with the newly added Dragon Dance from Heart Scale and you can also have a good sweeper. It's good base 100 Speed for a Ground type is amazing and its passable 100 Attack with high base power moves can do the job easily. The only downside I can think of is that it obtainable from an obscure trade that many people wouldn't want to go through the grind to complete.
Honorable mentions:
6,1 Tyrantrum: It works exactly like Flygon with a worse typing but more power! It comes a lot later than Flygon and has an even grinder event to complete than Flygon. I'd say you can use either one based on your preference, but I find Flygon a bit more consistent.
6,2 Arcanine: Guaranteed encounter and very early. It can already come with a good egg move but since most people are going to breed one, you might as well try to get that Howl or Body Slam for early game. The biggest downside is that you have to get stuck with Growlithe until lvl 45 for Flare Blitz, in which case you can feel the struggle for power until you get it. Also, Blaziken exists which is straight up better, but in case you didn't pick Blaziken Arcanine is the most consistent Fire type to use.
6,3 Ampharos: Probably the best Electric type you can use. It comes early, has good lvl up moves and evolves to the final stage withing the level cap you are the moment you get it. The downside is that it's slow and you have to rely to Static or Thunder Wave to outspeed pokemon you are suppose to be able to beat. If you are not going to use the event Emolga, please use Ampharos until you get access to Magnezone
6,4 Hariyama/Medicham: Hariyama is one of the earliest Fighting types available that it actually carries its big weight throughout the whole game. Evolves pretty early and has enough bulk to take hits and hit back hard. Medicham on the other hand it's a bit of a different story. It has great coverage with elemental punches and Zen Headbutt from Tutor but what you want to select with Medicham 90% of the time is High Jump Kick! Pure Power is stupid and HJK deletes targets than the speed of light!
Honorable mentions part2:
This part exists just to mention the 5 best pokemon that exist in Reborn and that's why you are not able to find them until very late game: Gyarados, Scizor, Excadrill, Aegislash, Metagross
These 5 pokemon are so OP that they literally break the game into pieces!