I already have my current supervisor writing a letter of rec for me, and will use letters from him for all schools I am applying to.
For schools that require a second letter, I'm choosing between a few options, as some of my most recent supervisors are not available. Here's my options:
1) Former supervisor for a long project ~4 years ago, previously wrote me a letter ~2 years ago, but no longer has it, and I think wouldn't remember too many specific stories to write about
2) Former senior colleague/mentor, last worked with ~2 years ago
3) Current colleague, a few years senior, but still at the same level as myself
On rank, number 1 is obviously the best choice, but I'm concerned a letter from him would be quite generic by this point, even though he was very enthusiastic to write one for me previously. Number 3 to me feels like would by far best meet most adcoms' instructions on who to choose as far as knowing me, my working style and goals/aptitude well. However, I am concerned about choosing someone is not a supervisor or formally senior to myself. Is this mitigated by the fact that I already have my current direct supervisor?