it's not a "what needs to be louder" question...
Start from the beginning. Build your rythym section foundation.
That's your drum and bass (sometimes rythym guitar).
Get your drums sounding like they are in a room, right in front of you. Close your eyes while you mix. bring up the bass guitar until it melds with the bass drum. Make sure you listen at different volumes. Very low mixing volumes helps decifer what tracks are most prominent in the mix, it more importantly which tracks are sticking out like a sore thumb. If it sounds good and balanced at a low volume, most times it will at 85db.
Next bring up the vocals just until they sound in front of the drums.
Finally bring up the guitars until they fill in the space that's left.
Use your EQ to "fine tune". This could be making sure two instruments are not fighting for the same frequency.
i.e. guitar is brought up last, but seems quiet because if you bring it up louder, you can't hear the vocals as well. Use your EQ to find the exact frequency the two are fighting over, and boost that frequency in one instrument (vocals), cut that frequency in the other (guitar). By cutting this frequency in the guitar, it can be a little bit louder without covering the vocal. But you have to use your ears, every mix is different and requires unique attention.
"Frequency fighting" needs to be check in all ranges: low, mid, high.
One other thing, Take breaks and refresh your ears. It's very easy to lead yourself into a slippery slope.
