I have an OG 5150, a Stealth 100, a Stealth 100 EL34, and a 50w 6L6.
If I'm recording and have access to all the boosts, pre-EQ, and post-EQ around, there's a good chance I'm going with the OG 5150. But if I'm playing live, both the Stealth 100 and Stealth 100 EL34 sound better to me straight out of the box so I'm taking one of those because it means less gear to haul around.
Random wankery on OG Peavey 5150 with a RAT as a boost and post-EQ'd to balance it out:
The OG is less filtered so you can do more with it, and overall it has something happening in the extended sub-lows that gives it a weight and thud that's really cool. However, overall EQ wise it's pretty centered / prominent in the stuffy low mids area, even with the Mids scooped pretty hard. The Stealths by contrast are more filtered and don't have the same sub-low thud as the OG, but their overall EQ signatures are much more balanced.
The Red channels on the 5150 III 100's are honky with the Gain set low because of how aggressively they're filtered, but setting the Gain high enough will help smooth out the honk, however by that point you're running a truly batshit amount of gain so be ready for that. You can apply your own boost or pre-EQ to smooth out the honk as well:
I think my favorite amp of all of them really depends on what day it is. Currently it's the 100w EL34. It has more present and crispier upper mids than the Stealth 100 without those mids taking over and making the amp sound small or narrow.
Also, the 5153 50w is one of the best deals in high gain tube amps that exists. Crazy good value. The Stealth 100's do sound different. They don't necessarily have more lows, but the 100 watt amps' lows extend farther down. Other than that they're pretty similar.
The Iconic 6L6 is also great, but I'd go for the Iconic EL34 between the two. The Iconic 6L6 has some harshness in the highs, and the Iconic EL34 seems to have addressed that.
These amps are not my go-to favorite amps for high gain, but the OG 5150 is as popular as it is for a reason, and with enough pre and post stuff, it can still sound as good as anything. The 5153's are really good in their own right too. I like them for what they are and I have no plans to get rid of them.