It's not possible to select one - every day there was something new and exciting! Seeing the everyday life of Thai, Khmer and Vietnamese people - along the roadsides, on the rivers, in Temples, in rural villages and teeming cities - and having the chance to visit homes through the kindness of our guides' families was a stimulating (and humbling) experience. Learning to cross the road in Ho Chi Minh City (with your eyes closed) has to be a bonus! Visits to temples, palaces, and markets as well as war-related sites, all contributed to my introduction to these countries. Not to mention tarantulas, hornbills and catfish! And seeing everyone in the group achieve their cycling and travel goals was a great ending!
Pack light - you spend a lot of time in bike clothes so don't need many different day/evening things. The laundry in Siem Reap is excellent/cheap. Padded bike pants are essential - and do take your own saddle/pedals if you are a regular cyclist. Getting them on/off bikes is no problem at all thanks to great support.
Take - small bottle hand sanitiser (most comfort stops are fine but just occasionally....); US$$ for Cambodia and Viet Nam in small denominations - almost everything costs $1 and any note over $20 is difficult to change; pens, pencils, small notebooks as gifts for children - you spend a lot of this trip feeling privileged, and so often you want to give something and these are much better than sweets; go for the single room option - I had a great room-mate but this is a non-stop holiday and if you like some quiet time, a single room could give that.
Odds and ends: insurance - you will be asked if you are covered for helicopter evacuation - most of us didn't know, so you might want to check that out before you leave. Wifi - every hotel we stayed in had free Wifi - so contact home is possible daily (as long as things work!). Cycling snacks - huge amounts of wonderful fruit, iced water, electrolytes were provided by support team so you may not need as many from home as you think.
The trip description/notes are accurate in relation to cycling times, conditions etc - slightly over estimate difficulty, hills, if anything, for a regular cyclist. But try to do some practice/cycling in preparation - if you are saddle-fit it makes the cycling easy and you will enjoy the holiday so much more.