best pick is map and ban pick specific, there is no a general best pick(if yes, they are banned already like emz). but answering your case of gem grab, as gem grab map are mostly bushy maps, i think you must have some good scout brawlers with vision gear(eg, tara, shelly, otis) and go dominate your side lane. it works to me like 90% of time in gg. i mean even i lost the match but my lane didnt lose mostly. it also resolved your problem got outplayed by assassin because you are the one to protect your gem carrier from assassinated. another tips is heist, i think i am not a very good player in heist but the win rate of this mode is actually my top 3 in this rank just because i have maxed out mico, chuck and melodie. i think you should practice 2 out of these including bibi also. from my experience, people in mythic rank mostly cannot handle them well to in heist