Best International: Tokyo-Haneda (HND), more specifically in the international terminal. The domestic terminals for JAL and ANA are on the older side, but they are better off compared to most airports in the US. Haneda's international terminal is new, spacious, clean, and offers great ramp views (including the spotting deck). It's on the large side in terms of walkability, but it's rather pleasant to stay in. My runners-up would be Seattle-Tacoma (SEA), New York-JFK's T4 (JFK), Seoul-Incheon (ICN), Oslo-Gardermoen (OSL), and Singapore-Changi (SIN).
Worst International: Los Angeles (LAX) overall. As much as some terminals (T2, T5, TBIT, etc.) are on the spacious, new, and airy side of things, some of the terminals haven't been updated since construction, or retain the same layout. T1-T8 were not designed for today with all the space TSA takes up and the number of passengers flowing through, leading to traffic into the airport, long lines at every terminal, and cramped and crowded gate areas. Runners-up in this category include Rome-Fiumicino (FCO), Atlanta-Hartsfield Jackson excluding Terminals T and F (ATL), Honolulu (HNL), London-Heathrow (LHR), New York-Newark (EWR), and Paris-CDG except for 2E and 2F (CDG).
Best Regional: San Jose, California (SJC). As much as it's on the smaller side, it's bright, clean, and modern. I appreciate that both short gate distances and space for TSA lines are present here. I guess it doesn't count as a regional airport anymore with all the international flights, but I guess it's regional enough considering the destinations it serves. Runners-up in this category include Bergen (BGO), Busan-Gimhae (PUS), and Portland (PWM).
Worst Regional: I'm going to have to award this to Santorini, Greece (JTR). Small terminal with little inside that can barely support a single flight, let alone multiple. It was fine to fly into but a pain to leave from. Other notable examples of bad regional airports include Osaka-Itami (ITM), Geneva (GVA), Kona (KOA), and Burbank (BUR).