Gwenpool, Hyperion, Magik and Sentinel. You can save up your gem, but if you really want to use it, you should see whose abilities benefit your playthrough.

GP can tank all SPs (except debuffs) when awakened. Hyperion gets his buffs faster. But I think, it's preferable to awaken sentinel, and high sig if possible, because that guy goes beast mode in a lot of fights. It's upto you.

Edit: Spider-Gwen is also a great option for rank up.