From what I read from lab reports the Mckinley has the most stable SSB while the Bearcat 980 tends to drift as does the newer Cobras.
Sadly Cobra doesn't even make a SSB radio anymore.
I have owned/own a few of the above mentioned radios.
There are actually 2 different 980ssb's. 1st rev and 2nd. The 1st revision ones where dogs and I think thats where a lot of the bad rap comes from. The 2nd is a solid radio that sounds
great on AM and good on SSB even with the stock slightly modded mic. And for $120-140 its a great value. I got a 2nd gen for my brother and have one in my jeep. Both have been flawless.
The McKinley is about the same performance and more solid looking display. Front fire speaker and an auto squelch system that actually works well. President also put out a complete service manual with schematics for this radio. Rare today. I traded it to a retired friend so he could get on the air. Hooked to an A99 he gets out great and has been having a blast talking to local OM on AM and shooting skip with it on ssb.
LincolinII+ does great on SSB and is all mode. If you have a ham ticket then this is a no brainier since it can do 12/11/10 all mode with PL/splits for repeater. The only thing I don't like is the auto squelch doesn't really work well. After running the McK for awhile I kinda got spoiled by that. Other then that its very solid and feature packed. It has SWR protection which comes in handy if your forgetful and switching mobile whips back and forth for different bands. I think the AM sounds a little blah. Its definitely no Stryker 955 but its also doesn't carry a Stryker price tag.
All 3 above mentioned have built in SWR meters and the McK actually has an automatic audible one so if your outside tuning just crank up the volume. When the tones become solid your good.
If you want to talk about drifty radios my old Texas Ranger 696f is one. Luckily it has a real 6 digit counter built in that I make good use of to stay put as it warms up. This was the 1st SSB radio I ever got and still use it to this day at home. Sounds and hears great.
To get back to OP's question. Best SSB rig I vote Stryker SR-955hpc. Those radios just have a presence on air.