Oh boy I can honestly say I've played both roles for many series so I'll just list the ones I'm a veteran and ones I'm a newbie:
Veteran: Pokemon, Fire Emblem, Legend of Zelda, Mario Series minus the classic mario bros, Legend of Heroes, FInal Fantasy, Xeno series, Kingdom Hearts, Animal Crossing, and probably a crap more.
Newbie: Hyperdimension, Tales of Series, Persona, Dragon Quest, Metroid, and again probably a lot more.
I'll start with Pokemon since I can honestly say it's not the main games that are my problem. Gen I was not the best gen at all and I could argue 3-5 were for many reasons. Gen VI is on par with Gen I tbh since both gens were very lacking in many areas. What make the RBY games so successful was how simple and fun it was using those cute little mons to kick butt. Most of us didn't even know what we were doing which just added to the fun since there was so much freedom. It was also something you could talk about to almost everybody, just like with Pokemon Go today.
Gen VI is my least favorite Gen (I'll admit I didn't like V until B/W2 which redeemed its faults...still a good gen though) because they took what we all liked about Pokemon and threw it in the trash. Sprites are gone after almost 20 years of buildup. Difficulty has been simplified and dumbed down (all leaders only having 3 weak Pokemon, really?), and the huge postgame is...nonexistant. ORAS just didn't hold the candle to what Emerald was either. That is combined with basically the removal of all those spinoffs which is what made Pokemon so good. A new gen was just an introduction to a whole big collection of stuff to come. Now...we've gone from V to VII with almost nothing but the MD series. I worry Gen VII will be just as hollow of a shell of a gen as VI due to all these changes.
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Now for Fire Emblem. Awakening and Fates are not terrible shitty games as people sometimes address them. Awakening had a poor storyline, but it's design was made so well to attract such a broad audience and its sandbox design was ingenious. Map design and story are lackluster but there's many diamonds in the rough with it to be worth playing once. Fates fixes all of Awakenings problems in terms of gameplay to the point it works very well. The removal of the weight system is the only glaring issue with newer FE games.
Then there's fates which is all over the place. Conquest was well done other than the low budget story. Birthright isn't too bad of a game either but it runs pretty much as an average game with just a bland story, and then there's Revelations where they did not quality check this game at all. It's just an example of when a company become overambitious and it stings them in the ass since there's so many inconsistencies. I can see why somebody would see it as their favorite since there's so much to do while vets could absolutely hate the changes that come with it.
Heirs of Fates is the exact difficulty a Fire Emblem game should be and with time and effort how well it can be pulled off. A lot of people such as myself expect a subpar story from FE that has it's moments and interesting characters, but it all grinds down to map design and gameplay. Quite a few fates maps actually are in my top 10 favorite FE maps. It's just the whole Hoshido vs Nohr is in reality Newbies vs Veterans in terms of what FE should really be but neither side is truly right.
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I'm just going to bring some closing remarks that how well games age really shows how good they are. Look at Legend of Zelda Wind Waker, that game is over 10 years old and still can compete graphically with modern games. Nostalgia is also a beautiful and evil thing when talking about older games since times and expectations change. I hate FFVII which is a lot of people's favorites since I didn't play it in that era. I felt is was far immature in the writing and the whole praise sephiroth just got old fast.
Nowadays, I try digging around for old gems and have really expanded my RPG gaming library to the point I can say I'm an RPG gamer. I still remember the days where I was so closed minded that I wouldn't play anything other than nintendo. Now I'm willing to pass up on the NX since I have plenty of other series to occupy my time. Just goes to show you that keeping an open mind lets you explore so many new games you'd never know you'd enjoy. Now excuse me while I go play my second run of Cold Steel before the sequel comes out in a month.