Without knowing your background, my take is -
1) Go for the recommender who knows you best
2) If your first choice manager isn't up to it, go for your current boss irrespective of how well he/she knows you, if he/she is enthusiastic and will give you a good recommendation.
3) Go with a current 'dotted-line manager' another former manager, client, senior colleague from an other department, in that order of options.
4) If nothing else works, maybe you can go to the most qualified recommender who knows/knew you in a professional or academic setting.
Well most schools want 2 recs one being current and other being past.
For current I have one who is chugging along maybe 1/wk - she is taking her time on current one because of the essay format instead of usual short answers so I am set on "current rec"
For past rec I have my boss/supervisor who is the problem right now, a tech lead who i didnt get along with (stubborn, spying on me etc.) from my last job. Then finally my summer internship 4 months with someone who is really nice and happens to be friend of the family as well. But that was back in my senior year and only 4 months so i am not sure... i am sure he will do them if needed...
So it comes down to this - should I :
A. wait it out - if my past rec is slow i will push some schools into R2 to accomodate.
B. give him some schools with deadlines - maybe push him forward...
C. use the internship rec as well to speed things up.
If you like my answers please +1 kudos!