The place you're going to want to start for investigating all record-lock contention issues is Salesforce's Record Locking Cheat Sheet.
It succinctly answers each of the questions from the OP:
- AccountTeamMember - yes
- Opportunity Line Item - when it updates the Opportunity, yes
- (Does sfdc lock opportunity here as well?) yes, as with Master-Detail relationships
- Task - yes on update/delete; on create, only if the Status is Completed and the ActivityDate is not null
- Contact - yes
- OpportunityTeamMember - no
- (Does sfdc lock opportunity here as well?) - yes
As for the final question...
Master Details: On insert or delete of a detail record, the master record is always locked. On update, the master is only locked if rollup summary fields exist on the master for that child type.
Lookups: On update of children, parent records are only locked when the Lookup value on the child changes. On insert or update, parent records are only locked "if [the] lookup relationship is not configured to clear the value of this field if the lookup record is deleted," which is the default option (clearing the value, that is).