My beloved T420's bezels are coming apart at the hinge. This has already happened once---I glued the plastic back together---but now is a very inconvenient time for it to happen again. So I'm thinking of upgrading.
For me, the most important factors are:
Portability - I have to carry my laptop with me on a nearly daily basis.
Linux support.
A good keyboard, preferably without the numpad.
An HDMI port.
Good battery life (~5 hours with active use should be plenty but more is welcome).
I'm thinking of going with the 8th generation X1 carbon but I'd like to hear your opinion.
Another T420 could be a good choice. They are fairly easy to find cheap and they are the last generation with the 7 row keyboard and non-chipped battery, iirc.
They are not cheap and the 7 row keyboard is quite overrated. Literally any second hand computer for the same price would outperform it in every metric. You also have the problem of choosing between a 13 year old battery or a 3rd party battery.
I've been fine with 3rd party batteries, especially without the auth chip of later models. I used an x220 til it finally gave up recently, and currently use a t520 and a t530. I do prefer the 7 row keyboard though it's not a huge deal. Sure, newer laptops are faster but so what? It's extra expenditure for performance that I don't need. When I want more CPU, I use a server or cluster.