I tend to battle my way through the games using two teams, and with 12 slots available there are some 'mons I tend to fall back on time and time again. They are strong, reliable, and usually quite conveniently available too. It's not like I use them in
every playthrough, but surprisingly often some of these find their way to my team:
Gastly is great at disrupting the early-game. It laughs at all the wild and trainer Pokémon that only have Normal-type moves to do damage with, it can cripple opponents with Hypnosis and slowly wear them down with Curse. Of course, its utility is limited by the lack of Special attacking moves, but at least it can use Hex to great effect. Haunter/Gengar can use a variety of TMs to patch up its poor level-up movepool, but many of those TMs are very late-game. And of course, it needs a trade to fully evolve. Still, Gastly is a solid addition to most teams in Pokémon.
Kadabra turns Pokémon into a P2W game: Psybeam/Psychic to Win. True, it has
one of the least diverse level-up movesets ever, but in 99% of cases you click the generic STAB move and win the battle. 105 Speed and 120 Sp.Atk is enough to put a dent in anything before it can react. You need to baby Abra for a while, but then it evolves and starts wrecking stuff with Confusion. Then it learns Psybeam at 21 and Psychic at 38, becoming successively stronger as you progress through the game. Once again a trade is required to evolve Kadabra, but it does remarkably well on its own.
If Magikarp is in a regional Pokédex, it tends to be available very easily very early. It has an awful babying period until level 20, but from that point on you've got a real monster in your hands. Switch in, Intimidate, boost a couple of Dragon Dances as the opponent struggles to damage you (Gyarados has only two weaknesses and really good bulk), then go to town with Gyarados' versatile array of damaging moves. It's so fun I can't help doing it over and over again.
It's the king of in-game, no questions asked. It resists almost everything, except a few (relatively uncommon) types that rarely pop up unexpectedly in in-game battles. Early-game, it blows holes in things with Sonicboom, and as the game goes on it gets better and better Electric options (and some Steel moves you will never use). Evolve into Magnezone or put an Eviolite on it and use it as a tank. It works well either way.
It may not be the most powerful Pokémon, until the very end of the game at least, but it tends to be available fairly early and Metronome is too fun not to use. You can get into some really fun situations with this one.
It has some issues with availability, but when it is available, I tend to give it a go. It's an Ice-type that's good in-game, and it evolves twice. What's not to love?
Diverse moveset, decent power, Moxie. You have the tools to hit almost every opponent, and then things begin to snowball. It's a shame that most in-game opponents carry few Pokémon, because Krokorok/Krookodile can pick up a lot of steam and then plow their way through almost everything if given the chance.
It may be rare, it may be weak for a long while, and it may be a tricky one to evolve since Dusk Stones are so rare. But the power Chandelure can put behind its attacks makes it all worth it. And of course, it is one of the best Pokémon designs ever.
One of the most solid Pokémon ever, period. Immense physical bulk, compounded by resistances to everything and three immunities (in fact, Honedge is only damaged neutrally by two types). Great Attack. Learns damaging moves of a wide array of types by leveling up. Nearly all of them are useful, despite a certain lack of STAB, and all hit from the right side of the spectrum. It is slow, but has a STAB priority move to make up for it. If you commit to Aegislash you have to change your play style a bit (King's Shield is practically mandatory in that case), but Doublade works well on its own too - even better with Eviolite.
You may also have noticed a common theme: Evolving twice. I really like three-stage evolution families, and am kinda bummed that Game Freak has become so stingy with them in recent generations. I like to have some "power level leaps" to look forward to throughout the game, which is part of the reason why I patch up my team with three-stage Pokémon. If Game Freak won't make more of them, I'll have to substitute with some we already have.