I think "Assignment" should not be a sorting option at all, "Assigned to me", should be a filter type.
Sorting:
- Last Reply Asc/Desc
 
- Start Date Asc/Desc
 
Filters (that can be combined all together):
- Assigned to me/member (me / username)
 
- Status (Open, Pending, Resolved)
 
- Priority (Low, Mid, High)
 
- User (Ticket Starter)
 
This is much more clear.
Then I can filter – for example:
"Assigned to me" + "Staus: Pending" + "Priority: High" + "Ticket Starter: Danny"
and 
then sort them by start date or last reply – asc/desc...
...so I can see all tickets started by User Danny that were of High Priority and are still pending while I pick that ticket first which is pending for the longest time...
OR (if I leave the filters away...) I can just sort all tickets for last reply and start date...
...that would be really really helpful.