what are the best attributes?
I feel like the one that puts a highlight over the mines is great.
I don’t know if the one that lets you sprint while reloading is any good and the one that lets you switch weapons faster seems useless.
of course its merc dependant, but what is the best combination?
Most people call them augments or perks.
Augments that strengthen the mercenary’s ability, increase their survivability and help out in every scenario are considered top tier. Examples are explodydendron, drilled, unshakeable, focus. Then there is Enigma which I nominate for the most useless augment generation two of loadout cards has to offer. Oh, regarding double time the community is split up, some love it, some hate it because with it you can’t cancel an ongoing reload by sprinting.
Some augments are clearly cut always good augments, like @Kirays pointed out already. But if I am honest, you should try the augments yourself by using them on a variety of weapons and mercs. You will develop an attraction to your loadouts that complement your style of play and just feel right to use.