All their normal albums anyway. A 10cc fan would be better off following their career than the post-G&C version of 10cc. It still sounds pretty much like the same band anyway.

L is as good as any of the first 4 10cc albums (even better than a couple of them, IMO). Out of all their albums L is essential for a 10cc fan. Freeze Frame is really good too, though it has a good chunk of tracks which fall under the category of "very interesting" instead of "very good" but I'd still say it's their 2nd best. Ismism and Birds of Prey are more inconsistent and introduce more dated production values, but still have good tracks and are worth checking out. They adapted to the 80s fairly well all things considered. They never neutered their personality or anything like the "real" 10cc did in the 80s. And their final album Goodbye Blue Sky is a really cool almost gospel-sounding album featuring a harmonica section. Sounds pretty different from their other stuff.

They also have two *not* normal albums, both of which are dubious quality-wise but again fall under the interesting-even-if-not-good category. Consequences is an incredibly ambitious triple album that includes some normal songs, sound effects, instrumental music, and spoken word. It was so important to them that they left 10cc to make it. But it's mostly a failure IMO. I'd say it's worth it for a curious fan to give it one listen all the way through, but beyond that I can't really recommend it. Best to just pick out the good tracks on initial listen to return to later.
The other one is History Mix Volume 1, where they collaborated with The Art Of Noise to turn a bunch of 10cc and solo G&C songs into a big extended 80s dance-medley thing. It's incredibly odd and extremely dated, but surprisingly entertaining. It's also notable as the album with "Cry" on it, which is one of their greatest songs ever and a worthy successor to "I'm Not In Love"'s epic-balladry. The fact that it's the only highly rated G&C album on AMG is baffling.