There isn't a single best database for biomedical conference abstracts. Try the following approaches.
1. Each of the 5 databases listed above (all but PubMed) contain some conference publications. Try Web of Science first.
2. Go directly to a meeting or association website to search for abstracts.
3. Search Google for your topic and require that the terms "meeting" or "conference" be used. For example, to find meeting abstracts where the phrase "tumor paint" was used, type:
(meeting OR conference OR proceedings) "tumor paint"
Why Use My NCBI?