Longest lasting as in run time between charges, or total life span?
Consider this regarding run time:
Often, radio users will want the highest capacity batteries by default. This can be useful for those that use their radios a lot, or are required to work long shifts without the ability to recharge. For "normal" use (5-5-90) during an 8 hour shift, highest capacity batteries are rarely needed. The large capacity will never get fully used, and the additional costs will not be recovered.
Probably a pointless statement if you are a hobby user, but consider it anyway. If you are a casual user, there is nothing wrong with going with the big battery if you don't mind spending the money. If you don't ever drain it down fully, you really are not getting the full value of the battery.
Finding a battery that fits your expected needs will probably save you some money.
I've got about 300 NX-410's at work. They will use the same battery pack as the TK-x180 radios, the KNB-33L. Our PD also uses the NX-210 as their VHF portables. Of those 350+ radios, every one of them uses the KNB-33L. Since anyone who relies on their radio will almost always make sure the battery is charged between shifts, those batteries work just fine, even for our PD officers. In reality, I've been able to get two full shifts out of my KNB-33L batteries. My radio is on 8+ hours, however I don't transmit much.
For kicks, I've got the KNB-32n and the KNB-43L. Both good batteries, but larger than what I need. The 32 is noticeably heavier. The other drawback to these is that they are physically thicker than the 33L, and thus can require a different carry case depending on the radio you are using.
As for battery overall lifespan?
I've been running the same Kenwood batteries for over 3 years now without issue.
It's important to note that Kenwood does not make the Kenwood branded batteries. I believe they are currently using Power Products, but I could be wrong.
As for Power Products brand, we do have a bunch of those in use, and haven't had any issues. I think last time I looked, they were $15 cheaper than the Kenwood brand for the 33L equivalent.