If you have to go 18650 and Li-Ion chem and if range is no issue with a 6P setup (I'd say 25 miles), Id go with Sony VTC4... Thes cell will run coll because they have low internar resistance (21 mOhm DC IR), thus very good cycle life.
If really you don't care for range for a 6P setup (think 12-15 miles) and you want inclredible cycle life and 10 year + lifespan, go with A123 18650 LiFePO4 nanophosphate 18650 cells (1100 mAh, yellow wrapped cells). Very very low internal resistance. Very good discharge rate. Will run super cool in your setup. But range sucks... On the plus side, charge rate can be quicker on LiFePo4.
But consider this 250W is such a soft setup, even running cells rated for max 10A discharge rate in a 4P configuration should last a good 3 years (just don't store at 4.20V)… So if it's range you are after Samsung 30Q (3000 mAh, 15A) or MJ1 would be a well balanced solution. You could also go Panasonic GA 3400 mAh 8A cell and they should still last 4-5 years.
If you can exit the 18650 format for something bigger format,
Then consider A123 MA1 LiFePO4 nanophosphate 2500 mAh 26650 cells. Best cycle life, but bulky and heavy for a not so great range (think 30 miles for a 6P setup).
I would personally go with the Samsung 40T 21700 cells. 30A rating means the have pretty low internal reistance (meaning they run cool and thus good cycle life) and at the same time have 4000 mAh capacity and don't tale so much space or weigth too much. Just, they are a bit pricy cells.
Think about it a 6P setup of samsung 40T cells at 36V, thats 864Wh of capacity and a max discharge rate of 180A, that your bike will baby up at 7A continuous and 15A burst.
With a 250W eBike, you really have a lot of options as most ebike battery will survive very long du to very low current stress. For range you can expec 10Wh per kilometer so around 80-85 km (50 miles) good enough ?
But to be frank, alhough the worst posaible option for cycle life, you could even get away with 6P of laptop cells for such a low power 250W setup (1.2A/cell continuous ; 2.5A/cell burst). Not that i would recommend that last option.
So choice is hard to make because there a just som many options. What would help is to define exactly what kind of range you need. Would you want smaller than 6P ?