My first favourite tier as a new player was 4. It had a perfect balance between its difficulty and my skill level, and finishing the maps was the most rewarding feeling at the time, as lower tiers felt too easy and boring and harder tiers impossible.
This however has been shifting as my skill improved. Nowadays I find fulfilment in getting 800 or 900 + points in the API; not just finishing the map. So lately I've been enjoying A LOT combining easy maps; tier 1 and 2. Flawlessly performing the route you'd planned feels so good, and it was a breath of fresh air when I found out about it.
But there's also one more thing I really enjoy, and it's paused technical gameplay, like technical_difficulties, kiwi's maps, etc. The fact that I can rest and reset my mind after every jump has its charm, and combined with tight moves instead of long bhops sections or super strafy jumps, it ends up being a really pleasant experience for me. I find tier 5 and a few tier 6 being the best ones for this kind of gameplay.
There's one more thing worth mentioning.This shift in enjoyment was heavily influenced by me starting to play in private empty servers. In public servers, the limited time I had was not long enough for a flawless run in easy maps, or a good one in technical ones. It was just enough to finish my tier 4 a few times and get an average run. That can quickly get boring, and it's kind of a cap in improvement. At least in my case, by transitioning from public to private I saw my skill skyrocket, and found even more enjoyment than before in things I didn't even try before.
I also think as my skill improves again, my tier preference will shift again too, as I'll be able to combo tier 3/4 and finish good runs in 6/7.