in my personal experience
sema: Strong GLP-1 only Slows the gut, makes you full fast af. You will still get hungry and have "food noise". Easiest to get sick on and has most sides. Stopped at 1.5mg every 7days cause I was getting nauseous during exercise and could not push myself on cardio or strength work as much as I wanted without feeling sick.
Tirz: weak GLP-1 and strong GIP. Heavy suppression of "food noise" and feeling of hunger, small but noticeable slowing of gut. I have almost forgot to eat on my high carb days because it works so well. The mental suppression to eat is more effective for me and I get no sides. Has a 5 day half life so I dose every 5 days.
Reta: GLP-1 GIP Glucagon. Same mental suppression as tirz with little gut slowing. Has a minor simulant effect at all times and makes me run hotter, sweat more during cardio, makes food taste worse. Just started on low dose so cant speak to higher doses for sides or effectiveness vs tirz. Half life is 6 days so that's how often I dose.
People have reported increased heart rates and arrhythmia on reta. My resting hr stays between 46-52 on all of them.
Some people stack them with degrees of success, if your foods all prepped and you just need to stick the show prep diet they all work on their own imo. Tirz just got the FDA approval as a weight loss drug so if ur gunna pick just one Id say Tirz, most effective with least sides and FDA says its good to go. Retas still in trials.