This is a cool topic, and it got me to think about where I’ve been. I’ve travelled for work and pleasure and love geography. Crossing a border is always a memorable event. I also love border quirks like exclaves and enclaves and arbitrary points and lines.
My foot/land vehicle/ferry border crossings: USA-Mexico, USA-Canada, Argentina-Uruguay, South Africa-Lesotho, South Africa-Eswatini, South Africa-Mozambique, Zimbabwe-Botswana, Botswana-Namibia, Zimbabwe-Zambia, Uganda-DRC, Uganda-Rwanda, Rwanda-DRC, Rwanda-Burundi, Singapore-Malaysia, Singapore-Indonesia, Hong Kong-Macau, UK-Ireland, Denmark-Sweden, Finland-Estonia, Spain-Andorra, France-Monaco, France-Belgium, Belgium-Netherlands, Belgium-Luxembourg, Italy-Vatican City, Austria-Slovakia, Ukraine-Moldova, Serbia-Bosnia, Bosnia-Montenegro, Montenegro-Albania, Albania-North Macedonia, North Macedonia-Kosovo, Kosovo-Montenegro, Montenegro-Croatia.
Most remarkable difference: Rwanda to DRC at Gesenyi-Goma and USA to Mexico at San Diego-Tijuana.
Longest wait: Ukraine to Moldova around 2 hours. This was in 2008-ish. That’s not really bad. I’ve been lucky.
Most surreal: the Vatican
Least noticeable: Ireland to UK.
Most memorable: Botswana-Namibia border on the Caprici Strip by boat with elephants in proximity
Best US-Canada crossing: by canoe on Cameron Lake in Watertown-Glacier National Parks.
Equator straddled in Ecuador and Uganda. Prime meridian straddled many places in London. North American and European tectonic plates straddled in Iceland, 4 corners USA, Continental divide at BC and Alberta, lots of most north/south/east/west points, many US state boundaries
Favourite USA exclaves visited: Point Roberts in Washington, the Kentucky Bend exclave, Carter Lake, Iowa oxbow. Alaska is technically a big exclave too.
Anyway, I could go in and on.