Honestly, I don't think any Kanto game has been particularly good, but the best worst option is FRLG. The Sevii Islands take the traditional Kanto experience and add the Sevii Islands onto it to give it a lengthy postgame campaign and more content. While HGSS does some of this too, it's not very fleshed out and even with HGSS adding some areas back in, there's still some areas that aren't as fleshed out as they used to be (The Rt. 7-8 Underground Path is still permanently closed, Mt. Moon is still a lot shorter than RBYFRLGLGPE, Cinnabar Island is still a mound with a Pokemon Center on it, Rocket Hideout is still closed, and Silph Co. and the Radio Tower are still restricted to the first floor). And the less said about LGPE the better, I'd rather just pretend that game doesn't exist because it offers nothing positive to the series, and especially nothing positive that isn't done better in another game. So FRLG wins less because of what it does right, but because of what it doesn't do wrong.
Really though, no game has entirely done Kanto justice. Game Freak's insistence on faithfully recreating every tile of Kanto to the way it was in 1996 and sticking to the terrible variety of the original 151 (as well as other things such as failing to adequately flesh out the plot and characters and not adding any substantive sidequests or gameplay features) has held every Kanto game back from being truly great. Kanto really needs a BW2 type of experience to put it on par with more recent Pokemon games. Not FRLG which tacks on a post game to an otherwise untouched experience, not a GSCHGSS which tacks on an equally shallow and archaically designed region like a conjoined twin, and not a Legends Kanto which would turn Kanto into a giant empty field (although Legends Kanto would probably be a step in the right direction). An actual BW2-esque reimagining of Kanto that uses modern design philosophies. Areas that aren't short, flat, blocky grasslands and display a variety of terrains and environments. A modernized Kanto Dex expanded with 100+ extra Pokemon that doesn't have terrible type varieties to the point where gym leaders/Elite 4 members have to spam duplicate lines or random Pokemon not of their type to pad out their teams. A story that's more than "Team Rocket is doing bad stuff, go stop them!". Characters that are more than hollow punching bags to battle and generic NPCs that do little to nothing. Lengthy sidequests that offer a large variety of extra content after the story is over. Gadgets and QoL features that actually bring Kanto up to par with more modern games. Until they make a game like that, Kanto will always feel mediocre and dated.