depends on the style u play and the bots u prefer.
If u already play a bot that does more damage then the others, then go for defensive avatars or delay reducing avatars.
If u play a bot which shots are easy, like a Frog or a Raon, then go for attack power. Since these bots with full attacking gear, can finish off someone pretty fast.
If u wanna make more and faster gold, then go for loot improving avatars.
If u are someone who "loves" to drop people off maps, then go for bungee power

so its really up to u.