There are two categories of cartomizers used by most ECF members: 510-compatible and Kr8. My guess is that you mean 510-compatible (with which I am less familiar), but let me start with Kr8.
From what I have read, some of the best tasting, vapor producing
Kr8 cartomizers are Vapor4Life's
Premium and
WOW ... and BloogPlanet's new
MaxX Fusion. No surprise; Leaford was involved in the development of both. FYI: These are more expensive than most Kr8 cartos ... and they are designed for use at 3.7V. Not sure how they will perform at 5V.
I see that NHaler carries
Kr8 Jumbo cartomizers with 3.0 ohms resistance, which is an optimum level for 5V (see below). There also are the Kr8-threaded CE2 cartos, which have gotten a lot of attention. But I find them to be a pain to fill. Cartomizer vaping is supposed to be
simple.
Talking about the
510-compatibles is more complicated because there are so many variations: regular 510, mega 510, mega eGO, CE2, drip tip cartos, clearomizers – many at various resistance levels. I don't know enough about these to comment (hopefully some other veteran else will chime in), but I can provide a guideline, i.e., the "right ohm" you requested.
I think somewhere between 3.0 to 4.0 ohms would be optimum. On 5V, that would yield between 6.3 and 8.3 watts. [6-8 watts typically is considered the "sweet spot."] The lower the ohms, the higher the watts (which is the power or intensity of the vape). I see that NHaler's:
Drip-O-Mizers are 3.5 ohms and
510 Mega Cartomizers are 3.4 ohms.
Clouds of Vapor carries 3.2 ohm CE2-style 510 cartomizers: both standard size and XL. I'm sure there are others.
IkenVape sells various cartomizers, but doesn't specify resistance level. [Surprising, since Isaac is known for various specific resistance atomizers.]