I actually love Kanto's layout. The way it prevents you from going back once you reach Cerulean (briefly, but still) gives it a little more forward momentum to me. I always thought the way the game leads you forward is pretty clever. It felt like it was giving you interesting things to investigate (the unidentifiable ghosts, the sleeping Snorlax, the party on the S.S. Anne), whereas events in other games leaned more toward obstacles you had to overcome. I also liked the choices you have in getting to Fuschia and Cinnabar, as well as the the way opening up Saffron essentially opens up the whole region to make the late-game experience a lot easier. I'm sure there's plenty of nostalgia talking, but I just feel like Kanto encourages exploration a little more, especially with some of the totally optional locations like the Power Plant and the Seafoam Islands. The Sevii Islands are just a bonus.
Johto as a region is a little less interesting, mostly just because the only time it isn't basically linear is when it requires you to just go backwards to Ecruteak City, and backwards with nothing new is definitely not the most exciting way to explore. Its landmarks are sort of a mixed bag for me, too. Stuff like the Lake of Rage and Ice Path are interesting and (at the time) original, but the Whirl Islands and Dragon's Den took potentially cool ideas and turned them into typical dungeons. However, I think Johto's cities are great, and the lore is probably my favorite from any of the games.
Hoenn's fun if only because it was refreshing from the sort of bland style of the previous two regions. Yeah, you get the usual fields and forests, but there's also a ton of ocean, a volcano, and some of the most distinct cities out of all the regions. Even though I actually prefer forests and mountains, you never really get bored in Hoenn since it's so varied.
Sinnoh, on the other hand, never really stuck out to me. There's a little variation in its geography, but snow and the swamp are really the only things we hadn't seen before. I liked that Mt. Coronet plays a big part throughout your entire journey, but that also means you have to hike through it multiple times. It also felt like I had to retrace my steps a lot, since a few places branch out in multiple directions. I definitely think Sinnoh would have been fun as a first region, but for me it didn't freshen things up enough after three generations.
I guess my final ranking would be this:
Kanto > Hoenn > Johto > Sinnoh