“BEST” depends on your requirements, however I’m a big proponent of building a clean farm (apply best practices and lessons learned from your 2007 farm) and then doing database attach upgrades. The drawback of this is that your farm configuration and SSPs are not migrated. So, for example, you won’t migrate your user profiles (you will re-build them) or content index (re-index) or BDC connections. There are mixed (hybrid) approaches that can address these drawbacks.
Probably the most important considerations here are to build out a deployment that can be “agile” from an IT perspective. The bottom line is you don’t know how your SharePoint usage will play out. Try not to restrict yourself to a specific topology and get “locked in”. Design your farms so that you can redeploy roles as usage patterns change.
<i>This question was answered by Spencer Harbar, Enterprise Architect at Microsoft and Dan Holme, Consultant and Trainer, Intellium.</i>