I have a choice between n only, b only, bg, bgn, and gn on my 2.4 GHz wifi and I would like to know which one would be the fastest for me.
BGN. You need N for speed, and B and G for range and compatibility with older devices.
If you still have any devices that only use B or G and you use them a lot, you will probably want to replace them (or upgrade their wireless cards) to something more modern. Slow devices using the network heavily gobble up valuable airtime. Contrary to popular myth, leaving B and G enabled on your AP is not harmful, and in fact it helps even your most modern clients stay connected at long range. Having B and G devices on your network occasionally is not a big deal.
I partially disagree with @spiff's answer (were it N vs AC I would agree). Forcing n only connectivity can increase your available bandwidth and thus throughput significantly.
There are 2 reasons for this - the one which seems to be overlooked is the addition of "wide channels" on 802.11n standard which greatly increases available bandwidth. By keeping backward compatibility with G (and B) devices you lose the substantial speed advantage of wide channels.
Another, much lesser reason is that when G devices do connect they waste bandwidth for everyone by transmitting at a slower speed. This is not much if a disadvantage unless the 802.11g device is actively transmitting or receiving.
I am also not convinced that 802.11B should be enabled - its an ancient standard and I struggle to believe that you can get better distance from it - I posit that - as a rule - devices designed around this standard are so old they can't have the sensitivity of newer devices. 802.11 N, with the addition of MIMI actively cancels echo/reflections and, by creating corridors must be able to reach further then 802.11B and G).
Can you mix 802.11ac Access Points with 802.11n Wireless? (although it seems to miss the point that 802.11ac is 5 Ghz only)
I would base this on your devices that you use on your network, if you know that all your devices use 802.11g or newer then it's safe to set it to g/n likewise, if you still have some 802.11b devices kicking around then keep it at BGN for compatibility. 802.11b has a maximum theoretical transfer speed of 11Mbps, 802.11g has a maximum theoretical speed of 54Mbps and 802.11n has a maximum theoretical speed of up to 300Mbps (When configured correctly).