If you ask me, "best frame" is nothing if you don't like playing it in the first place. Do you like tanks? Casters/ability damage frames? Weapon platform frames? There's also many variables introduced via Helminth. And the best frame won't do shit if you don't mod it right and don't play it right the higher you go - higher levels increase reliance on shield gating, staying mobile and things that let you straight up ignore damage.

Here's some things I do like a lot to go for myself though but I can take most frames currently and get them going for a long time in SP:

-Mag - very easy shield gating, elevates projectile weapons to great heights

-Zephyr - can ignore most damage, has incredible crowd control coupled with damage amplification

-Xaku - yoink gun be skelly, full defense strip and scaling damage

-Nezha - can tank for a very long time and isn't hard to make into someone who can tank and also shield gate

-Yareli - slightly more gimmicky water Nezha

-Ivara - when I want to snipe heads, camp a spot or roll around

-Gyre - what if World on Fire but electric and you can use Pillage to make it never fall off and survive too

-Grendel - UWU. Can shield gate, can tank, can strip armor.

-Citrine - well I'm a big status stan and she got this plenty

-Baruuk - punch, tank, with Pillage can go 5ever

Revenant is immortal, can be built for literally deleting health bars or for being an immortal platform for weaponry. Gauss, a zoomies full package boi. Gloom Banshee keeping enemies crowd controlled, coupled with armor strip and incredible damage potential. Garuda with endless slashes. Ash with his Disruption popularity.

And then there's Octavia which I can't play for long before I get bored off my ass but she's able to go ad infinitum especially if you give her an armor strip.

Even with endurance your "best" picks depend on which mission type you're doing specifically. Your Survival aoe DPS frame won't really cut it in a long Disruption, and your single target CC Disruption frame won't really cut it in a long Void Cascade, etc

In a Survival you get the most flexibility as you only need high enough KPM to keep up life support, so it's hard to suggest the "best" frame. You'll want DPS to be your main focus, so frames like Khora, Garuda, Saryn, Mag, Baruuk are great choices. Frame survivability can be easily solved with a bit of CC and using the environment - find a corridor or a side room for a camping spot to limit enemy LOS and gunfire. If going beyond lvl 1000 (which is a good few hours in) you'll be mostly forced into shield gating builds, but health/armour tanking and damage reduction can get you pretty far, so you can use something like Nidus for a good long while without too much stress

In Disruption, the things you have to play around are the energy drain and enemy Toxin weapon Conduit modifiers. Both of these absolutely fuck over shield gating builds, and once you're far enough in, health/armour tanking falls off anyways. You can brute force through both of the modifiers with Vazarin dashes, but if you want safe and reliable picks that don't rely on Vazarin, then Revenant and Zephyr shine for their Mesmer Skin and Turbulence. Stealth frames like Ash or Loki avoid all risk of enemy Toxin weapons (other than crossfire), and can Vazarin dash through an energy drain round to drop an energy pad at the Demo. If you're confident using Vazarin to cover you, you can use literally anything, and some great picks are frames like Xaku and Mag. But being confined to Vazarin can restrict build variety a bit (like using Unairu for armour strip)

In Void Cascade, you have an extremely fast mission objective you have to keep up with, while being constantly harassed by enemies. You can't set up a safe camping spot like Survival, and you can't set the pace like Disruption. You can bring anything with enough damage output to deal with Thrax and trash enemies, but since the mission dictates where you have to be, you can often find yourself in places with little to no cover, and it's very demanding on shield gating builds. This again means that Ash, Zephyr, and Revenant are your "best" picks for survivability and keeping on top of the cycling objective. You can bring Vazarin, but it doesn't bring any damage buffs like Madurai or Unairu to help with killing Spectral Thrax, so when trying to put together a build that covers all bases you can find yourself much more restricted than in Survival or Disruption

Condemn Banshee is my fav. Fullstrip, massive damage vuln, great CC, your friends will love you. Pillage Gyre is amazing if you invest some red shards in her, mine's currently sitting at 430% strength.

Garuda, love the invicibility with 4, constant power regen with 3, scalable damage to enemy level with 1, subsume 2 as you'll have the mod that gains you life when you spend energy. Innate slash damage. Bump up cast speed, become unstoppable in a room full of enemies. I do love playing as Atlas with the free punch augment tho, but struggle to pay enough attention to rubble.