When Evangelion was first broadcast, it was quite unique: while the OVA boom of the 1970s and 1980s produced a lot of innovative and experimental material, the end of Japan’s ‘economic miracle’ (along with fallout in terms of the bankruptcy of the first-generation toho-spinoff anime studios in response to the relative economic failure of the absurdly expensive Akira, with which almost the entire crop of anime studios were financially entangled) meant that the first half of the 1990s represented a return to safer and more formulaic shows. TV anime had never gotten terribly experimental to begin with (Belladonna of Sadness was an OVA, as was the Devilman remake, and while Ideon was a TV anime, the famously dark, gory, and psychedelic ending, Ideon Be Invoked, was theatrical-only). GAINAX was really the first anime studio to be fully independent from Toho & the Astro-boy lineage, and Evangelion was their first fully-independent hit (since their earlier stuff had largely been either OVAs like Gunbuster or collaborations with Ghibli).
Anno is like the Quentin Tarantino of anime: he has an encyclopedic knowledge of the history of the medium & a deep love for it, resulting in everything he touches becoming densely layered with references.
When Evangelion hit, it was like when Pulp Fiction hit, only more extreme because of the relative dearth of experimental anime for the previous five years. It had originally been broadcast on a time slot intended for young teenagers, but when it got moved to another timeslot where the audiences recognized the references, it did much better: these older viewers had been starved for interesting shows, and they were suddenly seeing this thing that was not only deeper and more interesting than everything else on television but also wore its love on its sleeve for everything they were already fans of.
The result is that Evangelion was declared the savior of anime — but such a thing happens every season, and we rarely remember such shows for long. The reason Evangelion remains interesting, twenty-five years later, is that it rewards rewatches & obsessive consideration the way few other shows do. Studio GAINAX was built by otaku & this show was built to have multifarious kinds of otaku appeal: a complicated lore for lore nerds, references for reference nerds, interesting mech & monster designs for figure nerds, great mechanical design for mech nerds, military & military history references for military nerds, and some of the most enduring waifus for moe appeal. On top of that, the story is deeply concerned with the psychology of people growing up in the wake of a society-destroying disaster without parents (i.e., the Japan of people 10–15 years older than Anno) & how their trauma manifests in social & personality dysfunction (something otaku often have trouble with).
Evangelion remains relevant & engaging today, in part because it is heavily responsible for the current anime landscape. Its unexpected runaway success spawned not just many imitators but a reorganization of the norms around the industry: ‘late night anime’, in particular the ‘noitamina’ block, became a thing because it became obvious that anime aired late at night aimed at adults & older teens with a darker, more formally-experimental sensibility could be not just profitable but extremely successful. We owe the existence of shows like Serial Experiments Lain and Cowboy Bebop directly to Evangelion’s success, but less directly, we owe the existence of all otaku-focused anime (and the otaku-centric marketing mechanisms — like short shows that make up their budget in sales of blu-rays and fan discs rather than by being extended toy advertisements, & shows that market themselves on the moe appeal of a couple attractive characters) to the lessons studios and production committees took away from Evangelion’s success.
But it also remains relevant & engaging today because it’s a damned good show — one that stands on its own without any understanding of the rest of the medium.
It becomes more interesting if you understand the history, recognize the references it’s making, and understand how it has influenced later shows in turn, but none of this is necessary to recognize it as a wonderfully polished, immaculately constructed show, full of heart but also incredibly technically proficient.
Several generations of anime fans have fallen in love with Evangelion, and then found that exposure to more of the medium has made their love for Evangelion only grow deeper (and shows that are popular with casual anime fans only rarely stand up to a more mature understanding — the exceptions are really just Evangelion, Akira, Cowboy Bebop, and Trigun).
tl;dr: Evangelion is a cracking good show, and is also important to the history of the medium, being both an homage to older anime & a turning point that drastically changed the shape of the landscape