For me...
For Better:
1)
Magneton (Crystal) - Very good defensive typing for major opponents, can spread paralysis to turn fights around, and hits very hard with Thunder + Rain Dance, with (lots of) item spam was capable of soloing Red by using his Pikachu to set-up and that too after outstalling his Thunder.
2)
Cradily (Omega Ruby) - Super tanky and gets even harder to kill with Stockpile, excellent offensive and defensive typing, and levels up turbo fast at higher levels.
3)
Nidoqueen (Crystal) - Very bulky and reasonably hard-hitting with good coverage, can be attained before Whitney if you get enough money and your Mom purchases enough items, was capable of beating Whitney down (forgot exactly how though lol), and was a great stalwart overall for my Nuzlocke.
Honorable Mentions:
4)
Bellsprout (Crystal) - Not gonna solo the game any time soon but their Grass-typing makes them a very welcome ally to Cyndaquil against the game's legions of Rock and Water-types, including the dreaded Anthony outside Union Cave whom he made mincemeat of (this guy has legit ended some of my runs in the past with his annoying Geodude's Rock Throw). Can sweep Bugsy while underleveled in spite of a level disadvantage against Scyther and a 4x typing disadvantage by setting up Growth to +6 and using Sleep Powder to set up as well as weaken Scyther - admittedly I needed a couple or so Potions and an X-Accuracy (in these games it makes attacks never miss minus OHKO moves) - and aided in catching mons as well as helping take down others like Croconaw who otherwise were quite annoying.
For Worse:
1)
Blaziken (Omega Ruby) - Died against Sydney of the Elite Four, thank you for missing so often HJK :( Still great though :)
2)
Stantler (Crystal) - Strong but hilariously poor when it comes to movepool access, needs a lot of babying to be happy enough for max power Return pretty much sucks at everything not called Attack or Speed and lacks either the powerful moves or the versatility to benefit from it. If you wanna use them, be prepared to baby them extensively with X-Items to get their full power Return. They can't even use Strength!
For Better:
1)
Magneton (Crystal) - Very good defensive typing for major opponents, can spread paralysis to turn fights around, and hits very hard with Thunder + Rain Dance, with (lots of) item spam was capable of soloing Red by using his Pikachu to set-up and that too after outstalling his Thunder.2)
Cradily (Omega Ruby) - Super tanky and gets even harder to kill with Stockpile, excellent offensive and defensive typing, and levels up turbo fast at higher levels.3)
Nidoqueen (Crystal) - Very bulky and reasonably hard-hitting with good coverage, can be attained before Whitney if you get enough money and your Mom purchases enough items, was capable of beating Whitney down (forgot exactly how though lol), and was a great stalwart overall for my Nuzlocke.Honorable Mentions:
4)
Bellsprout (Crystal) - Not gonna solo the game any time soon but their Grass-typing makes them a very welcome ally to Cyndaquil against the game's legions of Rock and Water-types, including the dreaded Anthony outside Union Cave whom he made mincemeat of (this guy has legit ended some of my runs in the past with his annoying Geodude's Rock Throw). Can sweep Bugsy while underleveled in spite of a level disadvantage against Scyther and a 4x typing disadvantage by setting up Growth to +6 and using Sleep Powder to set up as well as weaken Scyther - admittedly I needed a couple or so Potions and an X-Accuracy (in these games it makes attacks never miss minus OHKO moves) - and aided in catching mons as well as helping take down others like Croconaw who otherwise were quite annoying.For Worse:
1)
Blaziken (Omega Ruby) - Died against Sydney of the Elite Four, thank you for missing so often HJK :( Still great though :)2)
Stantler (Crystal) - Strong but hilariously poor when it comes to movepool access, needs a lot of babying to be happy enough for max power Return pretty much sucks at everything not called Attack or Speed and lacks either the powerful moves or the versatility to benefit from it. If you wanna use them, be prepared to baby them extensively with X-Items to get their full power Return. They can't even use Strength!
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(Black): this one is surprisingly good, even thought it's so painful to level up and once it evolve it learn like zero moves this monke can learn many TMs and become like army swiss knife mon, it even save me from being swept when facing drayden dragon dance team, (your sacrifice will be forever remembered Rain).
(Fire Red): after losing my only pschic/special counter snorlax to saffron gym trainer destiny bond gastly, i quickly scour through many other option and find hypno to be pretty good, with it's SpDef and stat i can go mixed with shadow ball/psychic combination which i find very good in run.
(Emerald): in my second run, i encounter slakoth in petalburg woods thinking it could be useful in later, i was wrong turns out Slaking is the MVP of that run even with truant it's bulk and power proven to be very valuable, it's able to save me from losing several time when facing last 2 gym leader with hyper beam, shadow ball, Eq, "filler".
(Emerald): during my first run at first i go with torchic as my starter thinking blaziken is pretty good for the run, turns out i was wrong while blaziken is decent enough, it was pretty frail and have a hard time in late game due to gym matchup and grinding place, even though i wiped out by winona i already can see it's difficulty of grinding blaziken for later gym and especially E4.
(Sapphire) After a brutal series of team losses between Wattson and Flannery, I was just about ready to call the run a bust or at least assume it would be a brutal slog. However, Spoink, and later Grumpig, pulled through heavily thanks to their solid bulk, decent abilities, and great movepools. While Grumpig was never a worldbeater, its ability to tank saved me during Flannery, Tate and Liza, and even Wallace. 
(Fire Red) So going in I knew Snorlax was strong, but actually using it? It's something I legitimately think is banworthy in FRLG for how ridiculously strong it is the entire game between its massive bulk, potent offensive stats, and insane movepool that can give you near perfect coverage between Body Slam/Shadow Ball/EQ (walled by Steel birds that aren't even in Kanto). Absolute monster all around
(Sapphire) You know, when I caught a water type for Flannery, I expected it to learn, you know, water STAB. Lombre didn't have that or really, anything going for it. It falling to a random encounter, while sad, did give me room to add Tentacool and I never looked back