I would personally pick the GW7900 from the list first. I absolutely love the lines of the 7900 vs. the other G's listed. Although, the 6900 and 5600 are iconic models within the annals of G-Shock history. The PRG240 is a very nice Pro Trek but it only comes equipped with their second generation sensor which is 70% bigger than the third generation and it requires a substantial amount of energy versus the third generation / current version of the sensor. If you want a Pro Trek I would recommend the PRW3500Y-4CR.
Now if your looking for a jaw dropping G-Shock to wear on your wrist I highly recommend the GWG-1000-1A3 Mudmaster.
Well hope my input regarding your question was helpful.
Thanks alot BACKBLAST72 for your comments, i do love that Mudmaster its on my to have list, and i agree the 7900 series is one of the best, i do love my 6900 though aswell its a classic design that i never get bored of.