Ampharos in any Johto game. Granted it's because GSC and HGSS don't have great mon variety, but Ampharos is one of the few good Gen 2 mons available early and usable for a Johto playthrough. You get it early as Mareep, even moreso in HGSS where you can grind it a little and give you an edge against Falkner, then it evolves in Flaaffy, and then Ampharos, and all around it's a usable mon despite its level up movepool limitations. Electric coverage helps immensely if you're using Chikorita or Cyndaquil, and natural Thunder Wave is a valuable asset to have in any situation. It really hits its stride later in HGSS when it gets Discharge and Signal Beam and is a resourceful attacker and team supporter. Did I not mention that it's cute?
Gardevoir in the Hoenn games is another one. While it's super rare, and sucks utterly as Ralts and to an extent Kirlia, it's an obligatory addition to my team any time I play through RSE or ORAS. Going from weak little Ralts, and then raising it into an incredibly strong Gardevoir feels so rewarding once it gets there, having a complete powerhouse who stays good right to the end. Calm Mind, Psychic, and Thunderbolt in RSE is a force to be reckoned with, and in ORAS it has a second STAB in Dazzling Gleam or Moonblast to help it even more. I love Gardevoir as a Pokemon which helps a lot, but it's even better when it's just such a rock solid pick in-game and one of the best early game Pokemon you can get (especially when the rest are disposable early game crutches).
Yeah I know. Staraptor. God tier regional bird, amazing Pokemon in-game that's available from the start, evolves reasonably early with the level curve, and kicks utter butt. Strong STABs in Return and later Brave Bird in the late game, Close Combat as coverage, useful ability in Intimidate? Not to mention Starly is cute, Staravia is pretty cute too, and Staraptor is just so damn badass and imposing. What reason is there NOT to use this thing? In DPPt it's also amazingly convenient as a free Fly user for in-game. So much usefulness and you just can't go wrong.
Luxray is such a cool mon. I know it's kind of not that good because it lacks a strong physical STAB, but it's a useful mon all the same. Intimidate is good, Electric coverage helps no matter which starter you pick (but especially so if you use Turtwig or Chimchar), and in Platinum especially it gets a few neat coverage moves in Crunch and in Pt Superpower in the post game, and it can also set up Light Screen which has saved my ass on a few occasions. I enjoy using it in DPP. It's still not that great of a mon in competitive, but in-game it's a useful and dependable Pokemon. And it looks rad af!
You can't go wrong with these two. You just can't. Krookodile especially has a god tier design, and Scrafty is obviously the best (if my username doesn't already get that across well enough), but awesome coverage and a god tier ability in Moxie makes them able to rip through teams like a hot knife through butter and snowball as their power goes up. And boy does it feel satisfying when you do it. Scrafty in particular can just wipe through 3/4 of the Elite Four no prob and has its uses against N and Ghetsis.
Aegislash in XY is an utter boss. Honedge is a cool Ghost sword, and its typing is godly, it has good firepower, early Swords Dance, and great physical bulk. It evolves late but it's worth it as Doublade is very solid in its own right. Once you evolve it into Aegislash, it's a boss. King's Shield shenanigans to alternate between super high offensive firepower and amazing bulk and great offensive moves and coverage to hit hard back in return. One of the greats of the Gen 6 roster.
Despite its issues in-game, it's one of the better early birds, and one of the most fun ones to use for sure. You can get the TM for Swords Dance early and between SD, Roost, Return, and Aerial Ace as Fletchinder this thing can put in work, and Flame Body is a clutch ability that can come in handy. Talonflame hits its stride later on though, as it gets Acrobatics and Flare Blitz and with Swords Dance this thing puts in work because it's so damn fast. It's a bird I can't go wrong with using in XY.
SM and USUM have a very high power level, and Mudsdale's tankiness becomes an incredible resource because of it. Stamina increases its already good physical bulk every time it gets hit, but this thing takes hits well and dishes them back hard in return, creating an incredibly resourceful Pokemon to have in an get all game guide for
medieval 2 cheats ios Alola runthrough. It's got good coverage with Rock Tomb and High Horsepower, and it can use Bide/Counter if you're feeling up for it. It's slow, but it takes hits, and in a region like Alola, you'll need that.
Motherfucker is one of the most rad early birds you'll ever meet. It's kind of alright as Rookidee and Corvisquire, but as Corviknight it's really great. Takes hits super well, can use Hone Claws or ulk Up to boost and hit hard. A set I used in SwSh with Bulk Up/Brave Bird/Steel Wing/Body Press proved a valuable resource in the game and was always a solid reliable Pokemon.
Just a few to name, haha. I'm sure other people have more, even if mine were pretty basic.