Okay, so looks like the best you can do is 750 BP from a 750 Karma character. I couldn't find anything that cost more BP than Karma. Start with human, raise all stats to 2, then grab every skill in the book (and chemistry and arcana, but no magic or resonance skills) which is 60 skills, and raise them all to 2 and spec them (minus exotic weapon and exotic vehicle), that brings you up to 676 karma spent at 1:1 ratio. Then you just grab more exotic weapon and vehicle skills at 2 until you reach 750. Drop a spec somewhere if you need to even the stuff out for 750 points.
Doesn't play well at all of course, but you do have a truest of true jack of all trades, with a DP of 4 in everything and alot of 6s floating around from specs.
I'm sure you could easily hit around 600 BP from a character that you'd actually play.
The best ratio for Karma is 1BP for 1Karma, but don't forget 35 points of disadvantages that give you 70 karma. (minus 37 karma free skills you loose in karma gen if you want to consider that point)
For previous examples, you can make better because for example willpower 5 make you loose points and it' the same with all skills that are under 1BP for 2 karma ratio.
I only compute max bp/karma ratio for classics meta-types so it's probably possible to make better with others. My example with a troll is legal in term of rules and use only 175 BP in attributes :
40 BP for troll (40 karma)
str 13 cost 85 BP or 350 karma (only maxed attribute)
body 10 cost 50 BP or 200 karma
int 5 is over 1bp for 2 karma ratio because of free knowledge skills (40 BP for 70 karma)
free skills give you 55 karma (1 skill at 6, 3 skills at 4)
gear 50 points or 100 karma
135 BP remaining in contacts for example (270 karma)
Advantages 30 points (excep attr 20. grim 10)
Disadvantages 30 points
Total 400 BP or 1085 karma.
It's possible to compare karma or bp cost but impossible to know if a character is better or not because "points" are nor representative of character effectiveness. The example I give is not a playable character, just a thing about I was curious. There's no point in term of gaming to know what is the maximum ratio you can make but it can give you hints to make your own idea about karma/BP systems.