I personally go for as much nicotine as possible. I use a lot of flavoring, and I don't want half my mix to be nicotine. I also like buying supplies as infrequently as possible, and 100mg is good in this regard.
Since DIY, I've gone from 24mg to 15mg. I've also started mixing for friends and family, and some of them use 24-18mg liquid. While 60mg can still make 24mg liquid, I still like the fact that I still use as little nicotine base as possible.
100mg also has numerical advantages. If I want my juice to be 15mg/ml, I know that 15% of my mix will be nic base. Easy as can be!
Any strength will work, you just need to make sure it leaves enough room for whatever else you want your mix to contain, including the PG/VG.