I'm also about to do a real hot take here: Using upscaled pics through emulation feels a bit like cheating. Where I feel conflicted is that surprisingly, a lot of the games mentioned here are not that hot looking on the actual PS2.
The best looking PS2 game hands-down?
It's a tie between Valkyrie Profile Silmeria & God of War 2. Both can run at a true 480p resolution at 60fps, while looking absolutely stunning all the time.
2nd place, you have Kojima games (MGS3 & ZOE).
3rd place is everything else
Problem is, a lot of stuff like Shadow of the Colossus, Silent Hill 3, Kingdom Hearts, Rogue Galaxy etc. are fairly messy looking. Between the sub-480i resolutions & framerates, it can get, idk, "not very fun".
Even my very personal favorites like the Fatal Frames, Xenosagas & .hack//gu suffer from this.
Doesn't matter how much you tinker with your TV, GSM & real upscalers, some of these classics just can't output that clean image that you want. But it really is on a game per game basis. Because when a game is truly 480i, it looks seriously sharp.
(I wrote too much. Just my thoughts about my PS2 relationship over the years...)