To start off, please dont grind till you are around lvl 34, cause that is when ur quests starts to slow down. Always complete ur quest FIRST (When you are 35 and below), grind in Air/Water. When you are able to grind well later on, I always do quest when im a few levels higher than it so its easy (E.g Do a quest given to u at lvl 40 when you are lvl 44-46).
When u get to 59++ and have ur Dragon's Breath, this is when you would want to start your zhen grinding. Solo grinding cannot compare to the amount of exp you get when you zhen. Though at the start it is costly due to the need for having MP charms, as u get higher in your lvl, you will kill faster and get more money for the same amount of MP used.
Stated below are the mobs i REMEMBER grinding on and never regretted. Obviously depends on your build and Eqs.
~Some Nice Grind Spots~
Abyssal Mystery (lvl34) (Water) <557,528>
Vipion Leaper (lvl35) (Wood) <254,464>
Wimple/Bloody Goldwing (lvl36/37) (Wood) <626,727>
Big Mouth pirahna (lvl39) (Water)
Loft Pterosaur (lvl40) (Metal) <612,826><627,869> <--- Good exp, easy kill
Vampiric Thresher Lord (Lvl40) (Water) <674,835>
Native Vipion (Lvl41) (Wood) <379,781>
Ramshackle Roc (lvl48) (Metal) <154,557>
--Ramshacks hit u more, u hit them more too using fire, have good drops and exp (Info giver : Ursa)--
Vipion Sentinel (lvl50) (Wood) ( Above Skull mountain )
Blood Thirsty Shark (lvl52) (Water) <598,976> <-- Love these as they are easy to kill
Terrestrial Foxwing (lvl55) (Fire) <663,995> <-- Love these too
Bloodthirsty Shark King (lvl55) (Water) <701,848>
UltraFin Protector (lvl58) (Water) <712,731><721,773>
Foxxwing Supreme (lvl60) (Fire) <435,377><500,347> <-- LOVE THESE THE MOST! Spent so much time on it with ESO.
Pyrofiend Immolant (lvl 67) (Water) <546,473> <-- Ground mob!
Wretched Hydrolace (lvl 67) (Water) <546,465> <-- Ground mob! Beside Pyrofiend Immolant
Minion of Alucard (lvl 68) (Water) <566,475> <-- Recommend this more than Wretched
Hydrolace. (Hurts more, but more exp!!! Much more in 1 hour grind)
UltraFin Supreme ( Lvl 88 ) (Water ) <275 856>
Many thanks to _makina_ and BrownFlamez for this info.