I didn't realize the main point of consuming coke was the taste.
Going along with the HFCS theory, Mexican coke is an option, as is
kosher-for-Passover Coke. If you can taste those, HFCS is the culprit; Mexican coke should be easier to come by this time of year.
I do find there to be somewhat of a difference between HFCS coke and cane sugar coke. Not enough that I'm going to make a big deal of it if HFCS coke is what's available, but I'm considerably more likely to get Coke at a restaurant if it's Mexican coke than "regular" American coke. Which in turn I'm more likely to get than Diet Coke, since the difference between aspartame and HFCS is larger than between HFCS and sugar IMO. Though it's less pronounced in soda than it is in yoghurt; get two Yoplait yoghurts of the same flavor, one in the light aspartame version, and one in the sugar version, and it's quite a stark difference*.
* I think they may have switched to sucralose in recent years, so it may not be possible to buy the aspartame version anymore.
I've also had sugar beet Coke; can't tell a difference between that and cane sugar, though I haven't done a side-by-side comparison.
I recommend Mate. Or ice tea.
I do second the Mate recommendation as a caffeination option. I keep both it and black tea available, though I enjoy hot tea more than cold tea.
Another option may be caffeinated gum, which is a thing nowadays. I've never been that into chewing gum, hence tea being my preferred mode of caffeination, but it's probably better for you than drinking soda all the time.