Your guide and his cousin knocking back a few Bia Saigon in - where else - Saigon, Vietnam. Image courtesy of Jake Robillo, used with permission.
I wouldn't tell you what beer you should or shouldn't drink; your choice of beer is a personal thing. I will admit to being extremely biased for my hometown brew, San Miguel Beer, but I'm more than willing to try other beers when I'm on the road. Southeast Asia's beers are, after all, some of the finest you'll ever have the good fortune to try.
Three I like, in no particular order of preference:
Bintang Beer - one of my favorite things to come out of Indonesia, and that's saying a lot. Bintang is just perfect for Indonesian food, something I simply cannot do without when chomping down on a nasi campur in Bali.
Bia Saigon - seems like a rite of passage when you find yourself in Pham Ngu Lao in the tourist district of Ho Chi Minh City; knock back a cold Bia Saigon with your travel buddy and talk about the fun you're about to have in the days ahead.
Beerlao - if anybody ever doubts that any good can come out of Laos, then this ought to shut 'em up: an easygoing little pilsner that surprised the beer experts and made the top of their lists.
(One beer I do not really care for: Tiger Beer. When someone says "Time for a Tiger" I actually hear "time to look if the place stocks San Miguel"; thankfully, not a few bars in Singapore actually do. Tiger Beer drinkers, sorry, just keepin' it real.)
Read my list of Best Beers in Southeast Asia for more details, or consult Thailand guide Suzanne Nam's related list of Thai Top Beers. The Thais, they too know their beers. Sawasdee-ka!