Best Cards for xp/time are 3* cards. period. a 4* or 5* takes waay too much.
Here is an example :
a 3* needs around 3-4 battles to get from lvl 1 to lvl 3. that means 10+20=30 xp points
a 4* needs around 9-10 battles to get from lvl 1 to lvl 4. that means 10+20+30=60 xp points
a 5* takes too much. do not level 5*
so you see my point. if you level 3* you get xp much faster. gg's