Sinnoh is really good in variety and seems to be the biggest, but not much was happening in the many cities. Probably my favourite region when it wasn't causing slowdowns.
Unova had many things happening in the cities. The only problem people really have with it is just the layout of the map. If it was laid out in a way that didn't look linear, people wouldn't at an eye at it.
Hoenn is the weakest due to water, but it's land section had lots of variety.
Kanto really had you travel around, more so than any other region. It's cities were interesting too. It is probably the second best region for me.
Johto is weak due to it really not having anything different and felt so condensed compared to Kanto. It did have some interesting cities though. I think Olivine City may be my favorite in the series.