I'm fickle - I love the one I'm with. I've had 4, 6, and X. The 4 had the most solid feel thanks to all that stainless steel, but i was happy to leave the small display and 30-pin connector behind. The 6 was a great leap forward in many regards - Touch ID, better cameras, bigger display. The X was a nice jump up from the 6 - Face ID, better cameras, more display area, water resistance... And I'm really looking forward to 13 Pro.
The same is true for my first Mac (Quadra 950), my first-gen iPad, and first-gen Watch... they all introduced me to the joys of those platforms and I hold no small affection for them, but they've been succeeded by newer, improved models. I can't recall ever wishing I could go back.