Dung question

If I'm leeching and get to my highest floor and reset do I need to rush the floors up to my highest floor to leech and get the best xp or can I just reset and go straight to floor 30 let's say

 

You'll want to rush floors 1-29 (depending on your level) and leech floors 30+
Once you have done all your floors reset your progress so you get the highest prestige. And continue this until level 120
But you can do your floors in any order during your prestige, as long as you don't reset before you have completed all your floors

 

What happens if I reset and left out 1 floor?

 

Nothing too bad, you will jusy get a small amount of xp less each floor on your next prestige (if you only missed one out it won't be too much).
Just make sure you have all floors ticked off before you reset

 

As per what Ojaab said, the dungeoneering experience you receive per floor is based on many factors. The two major ones are your prestige and what floor it is. If leeching you should do probably your top 10 floors, and rush the other ones in c1 smalls so when you reset you have the highest prestige possible to ensure high experience gain.