I personally think the Boge Soft-tip cartos are great at 3.7V and also 5V. Mine are about 3.3 ohms, although they are sold as 3.0 ohms. Haven't used them higher than 5V, but my guess is they will do well, perhaps a bit hot. The CE2s, in my experience, even though they are also about 3.0 ohms, are too hot on 5V. This is likely due to the design, as the heating coil is close to the mouth piece and tends to spit really hot
juice on the tongue. The CE2s, however, are killer on
low voltage. Giving my regular 510 batts (3.1V) a new lease on life. In fact, I find even 3.7V with the CE2s can be a bit hot.
There are also regular-fill 2.5 ohm cartos, but these are too low in resistance for me on HV. Of course, these are just my personal opinions.