Advantix plus Interceptor for my GSD (although we've been using Heartguard while Interceptor has been off the market).
Advantix has been completely useless on my mutt puppy and we battled fleas for the better part of last year. For him we just landed on a winning combo of Revolution and Comfortis (we switch off every 2 weeks). Both provide flea control but Revolution has the heartworm preventative as well. Revolution is topical and Comfortis is oral. I think his case is a little extreme, I haven't heard too much about many dogs having resistance to Advantix, I guess we just got lucky. Ha!
Drs. Foster and Smith has a really great checklist of which meds work together or could cause problems.