As you might guess from the username, I love the pokemon ranger games. They were the first ones to make me really feel "Yes, this is who I'd like to be in the Pokemon world". The emphasis on keeping pokemon in their natural environment, and not making everything a battle, really felt right to me.
I played the original game first, and got stuck on the Salamence boss fight. But I still wanted more Ranger content, so I switched to Shadows of Almia. The use of a progress bar rather than a number of loops really helped as an anti-frustration feature. I eventually felt confident enough to give the original a try again and was actually able to beat it, before returning to finish SoA, and love both.
I will admit I dropped off at Guardian signs. I wasn't a huge fan of the changes to the capture system. I didn't like that assisting pokemon were now placed in the field (and thus vulnerable to being attacked, rather than just a one-time boost) and that I didn't have direct control over when my partner pokemon would feel like doing their assist move. And while I like the signs mechanic, it felt a little weird to be literally riding around on legendaries like they were my personal bike.
Between the three, Shadows of Almia definitely felt the best, gameplay wise, but I still have a personal soft spot for the setting of Fiore. And while I could see how it would frustrate others, I kind of liked not being able to take pokemon out of their areas. It fit with the environmental vibe of the rangers, and it meant that whatever I needed to solve a puzzle was somewhere around me. I also preferred the original's choice to limit to one partner pokemon. As convenient as it was in Almia to be able to choose a specific Type for my partner, I felt less emotionally bonded to my farm full of partners.