Realistically probably something not too long and something with a decent English Dub just to wet your toes. Also something that does not have excessive self monologue or people talking about their motivation and beliefs out load too much. A lot of anime has this and it can be weird for a western audience. I would not bother with Attack on Titan for a while yet.
Armin from Attack on Titan probably spend multiple 5-10 minute long sessions talking to himself about how he's letting everyone down, he's a failure etc etc. The titular character from Naruto can often spend similar amounts of time talking about his own positive motivation or directly to other characters and it can be quite weird when you see it for the first time and can make getting into a series quite tiring. While I love Naruto and Fairy Tail they have a big potential to burn people out or put them off Anime.
So firstly think of what genre's you'd be interested in, Romance, Sci-Fi, Action, Fantasy etc and start from there. But here are my personal recommendations.
Full Metal Alchemist - Any of the series or films are good, is a great place to start if you don't mind a bit of fantasy, kind of an industrial era with magic.
Outlaw Star - A good action packed sci-fi series. Only a single season so you don't haver to invest too much into it. Very much firefly vibes if that is your kind of thing.
Cowboy Bebop - Similar to Outlaw Star, again a single series, but more stylised and cool. Probably the better out of the two but less action packed and not quite as funny.
Tenchi - A romantic series, part of the Harem genre. Many series and films, but I'd pick up Tenchi Universe. While nothing ground-breaking it's a incredibly popular classic Anime/Manga and it'll set you up in an extremely over the top way on how Anime does romance and comedy. Again does have Sci-Fi but nothing over the top so if you don't like Sci-Fi it shouldn't affect your viewing pleasure.
Bleach - Probably the best Shonen for a beginner, if you want to get into a show with tons of episodes this is a good place to start. Tons of massive Swords, evil Japanese spirits etc. The quality drops as it goes on but is a really great show to see if Shonen is for you.
Alternatively
Hunter X Hunter - A great shonen, probably better than Bleach objectively. Not overly exciting to begin with but it just oozes quality. I wouldn't bother with the English Dub here though so only watch if you like subs.
Next two incredibly popular series that are fairly newish
One Punch Man - Just a funny take on the Super Hero Genre, doesn't take itself too serious and extremely relaxed viewing!
and
My Hero Academia - A more serious take on the Super Hero Genre (but still not super serious), you've probably seen it everywhere and that is for a reason.
Finally if you think giant robots are your thing, one of the Gundam Wing series would be quite good. My personal favourite is "Mobile Suit Gundam Wing", which was responsible for the series success in the west. However "Mobile suit gundam iron-blooded orphans" is arguably much better, newer, looks prettier and deals with more mature topics such as child soldiers and neo-colonialism. "Darling in the Franxx" is a newer popular Mech series, but not really sure it's for starters.
To end things, anything by Studio Ghibli. Spirited Away and Howls Moving Castle are just staples anyone has to watch even if you're not into anime!