I've had all 3 - Kemper, AxeFX Ultra | II | II XL | FM-3 and just recently sold my SYN-1 and half my modules to a member here.
The Kemper was cool but I don't really get using it if you're not profiling your own amps. There are some good profiles available out there, but they are dialed in on other people's rigs and never really sounded right to me.
Fractal is great, if you need all the options and effects. Their modeling is great and I sold two amps because it nailed them (EVH 50w and a Tremoverb). However, I just don't find using them to be fun. Even though the current gen is really convincing and sounds great, I just don't like that you have to press buttons and dig through menus to change simple settings. It's not as tactile as having the amp with all the EQ knobs in front of you. The effects are amazing though and worth the cost of admission alone if that's your thing - I don't really use effects.
The Synergy stuff is really good. I bought it because the thought of replacing my Mark IV with the IICP module for a lot less sounded great, plus I really wanted to try the Savage module because that's been a bucket list amp for me for years, but current prices are bonkers in the US. I had the Deliverance, VH4, Savage, BE, IICP, SLO, Ultra Lead and Uberschall.
My favorite, by far, is the Uberschall - I had a Rev Blue and a Twin Jet - the module sounds better. That said, I can get pretty close with my Bogner 101b. The Deliverance I was pretty disappointed with, it sounded similar to my SigX, but the mids were totally wrong and it lacked the hifi sound of the real thing. The Ultra Lead nailed it and I sold the SigX because it was close enough, minus the great clean channel. The IICP definitely sounds like a Mark, but it's not as aggressive as the IV. The clean on that module is, quite possibly, the best clean I've ever heard out of an amp. The VH4 I was pleasantly surprised with, it sounds way better than I remember the amp being. The BE was okay, I was hoping it'd get heavier, but I wasn't super excited about it. The SLO sounds like an SLO. Would be better if it had separate EQ controls so you could dial in two different tones. The Savage was a huge disappointment, it sounded anemic, lacked low end and wasn't as aggressive as I expected it to be.
Since I ended up liking the Uberschall the most and can get really close with the Bogner I already have, I decided to sell it. Before I did, I profiled it in ToneX, though, and it's equally as awesome.