The best time would depend on the person. Normally I'd advise you to take finasteride and minoxidil and stabilize your hairloss. However, you say that those only made your hairloss worse.
Question....how do you know it made them worse?
About a hair transplant, just think about it. Let's say that you get a 3000 graft hair transplant. What happens if you end up balding to a NW6 pattern (old man horseshoe, if you're not familiar with the Norwood scale)? Then you'll have a bald head...with 3000 random hairs still on it. 3000 might sound like a lot, but it is not NEARLY enough for coverage.
Now you'd have a strip scar, which means you can't shave your head without a nasty scar showing. Best case scenario would be that you got a FUE transplant and could still shave your head, but then that basically means you wasted the 6-10k on your transplant :-/
Balding is a b**ch, I know. I'm just glad that finasteride is working for me at the moment.