Definitions, according to me:
isekai - someone, usually the protagonist, got transferred to another world
trash isekai - there are about a dozen of extremely similar anime released each year, each being isekai or adjacent
if you watch one, you’ve seen all of them
their defining traits are:

• nice protagonist with “useless” power that is actually overpowered that they unleash when angry
• (hyperbole) protagonist is the greatest, noblest, and fairest creature walking on that world, everyone else is wrong or evil
• harem of archetypes (authors don’t even try to make their characters different) the protagonist is usually not be aware of
• plot where protagonist solves problems on his own in max three episodes
• (often) rpg mechanics that just go “his power is over 9000” with no people planning builds or significant effects on the world or…

Examples include: Tsuki ga Michibiku Isekai Douchuu, Genjitsu Shugi Yuusha no Oukoku Saikenki
While they are called this way, I know a person who watches them because they don’t have to focus and still have some dumb fun.

battler - anime focused on battles, I’m pretty sure Dragon Ball: Z falls into this territory, but I’m basing that on two random episodes I’ve seen and everything I’ve heard over the years
season - most anime air in 12 or 24 episode parts, 1 episode per week, starting and ending within 2 weeks of each other, hence they are grouped into 4 seasons per year
VN - visual novel, basically a book with pictures and sound; you’d be surprised how many get adapted

I’ll begin with where NOT to start:

• Gintama - it’s a parody referring a lot to anime, manga, and japanese culture from its age, although it has a Donald Zuramp joke too; well worth trying once you get to know some classics, don’t even need to finish them
• KonoSuba, Isekai Ojisan, Meikyuu Black Company, Shinchou Yuusha - they parody specifically trash isekai, watch one or two before jumping in
• Mushoku Tensei - perverted 40 year old mind in child’s body, even I had problems watching the first two seasons with over 150 titles (counting seasons individually) watched back then

Decent starts:

• Black Clover - good, long battler (170 episodes and counting); share my pain of waiting for continuation
• Dr. Stone - finished last season, mostly kept the same level throughout, but over time we got less science stuff; whole thing is about speedrunning from stone age to space age
• Dungeon Meshi - extremely good worldbuilding and the story gets better over time
• Helck - comedy and dark fantasy
• Shiguang Dailiren - the best anime about time travel
• Gyakkyou Burai Kaiji: Ultimate Survivor - two seasons of extreme gambling, CW: self-harm, torture
• Re: Zero - going back in time after you die isn’t the most pleasant experience when you have no other powers; CW: gore
• Ansatsu Kyoushitsu - comedy with good plot, some action and drama
• Initial D - fear not the ass 3D and potato faces, for you come for the music and racing, then stay for the story
• Mairimashita! Iruma-kun - good for calming yourself down, 4th season currently on air
• [Paripi Koumei](Paripi Koumei) - grand strategist acts like a grand strategist to help a random girl with her musical career

Comedies, the weirder the better:

• parodies from the first section
• Asobi Asobase - you’d think that it’s a slice of life; what a fool you are
• Kyuuketsuki Sugu Shinu - the vampire dies in no time, then comes back to life, then dies again because a dog barked too loud
• Mattaku Saikin no Tantei to Kitara - just watch the trailer; slight focus on parodying detective stories, unless I’m an ignorant and missed too many jokes
• Saiki Kusuo no Psi-nan - super powers are such a hassle to have
• Seto no Hanayome - my bride is a mermaid, and her mafia family wants me dead
• Youkai Gakkou no Sensei Hajimemashita! - guy who is scared of mere thought of human hooligans is forced to teach in a ghost school

Other recommendations:

• Mob Psycho 100 - three seasons of absolute cinema where super powers don’t make you super happy; give me back the evil kazoo theme
• One Punch Man - as above, it’s even by the same author, but uhh… hope for a remake of the third season… and maybe second (I like them anyway, 'cause I’m a fanboy). First is the best it could be, though.
• Hellsing Ultimate - BLOOD
• Teogonia - battler with an interesting world, isekai element shoved in to justify protag’s overpowered ability
• Steins;Gate - another good anime about time travel; I’ve played the remake, but anime is generally praised; avoid Chaos;Child like fire, once again only played the VN, but others say anime is bad compared to VN, and I say VN is shit for all the unnecessary gore and annoying protagonist, even if the concept itself was interesting
• Undead Unluck - once again a battler with an interesting world, this time without isekai
• Sonny Boy - you watch an episode, then spend a week trying to understand WTF did you just see
• JoJo - it really is its own thing among battlers, personally didn’t like it, but it’s popular for a reason
• Under Ninja - battler where weapons are actually lethal
• Ragna Crimson - high highs and low lows, battler with interesting pair of main characters
• Monster - it’s similar to western media, even the events start in Germany
• The Fable - assassin needs to hide for some time, goes to work for the lowest wage, problems arise in the neighborhood; very well paced, has some really tense moments
• Gachiakuta, Jigokuraku Jujutsu Kaisen, Kimetsu no Yaiba - good battlers
• Tensai Ouji no Akaji Kokka Saisei Jutsu - best attempt at grand politics I’ve seen to date; still too simplified