The easiest by far to me is the Kanger T2, - you can buy replacement coils for it as well.
It is however specifically made for the eGo battery so if you have a different PV, you will need a 510 adapter. I have removed the coil from the top of the clearo, put a new one in (still with liquid in the
tank, let it soak for a minute, and right back to it. you don't need needles/syringes/turning it upside down etc. Just tilt that
juice bottle and squeeze. Also I find the
tanks last for months and months and....
(NB - I don't vape citrus or highly acidic juices however, so if that kills the idea for you, you'll want to look at pyrex or glass).