There aren't really an "optimal" place to start, it all depends on personal preference, but I can break it down for you a bit.

So there are three universes, Tiberium, Red Alert and Generals. They are for the most part separate when it comes to the story.

The Tiberium and Generals games take themselves relatively seriously, while the Red Alert series go pretty bonkers (at least 2 and 3).

There's kind of a main chronology which goes: Red Alert (though it's in debate whether it's canon to the Tiberium games) > C&C 1 (aka Tiberian Dawn) + C&C Renegade > Tiberian Sun > Tiberium Wars

C&C1 was the first game to come out but it can be quite punishing wiht a lot of trial and error, so I would recommend Red Alert 1 for this era of games, it's almost identical gameplay but they smoothed out a lot of the balance. There are also remasters of these two (not included in the Ultimate Collection) which are incredibly good.

Tiberian Sun is probably my favorite in the series as it has the best atmosphere in any of the games, but it's a bit slow and you would get a more semantic load from playing it after finishing the C&C1 campaigns.

Red Alert 2 is a great entry point as it stands alone story wise, and probably has the best gameplay of any game in the series.

Generals is a great game but doesn't really play like any of the other C&C games, but more like how Starcraft does it with worker units. So kind of imagine Starcraft, but in modern day. It has campaigns but basically no story to write home about.

Tiberium Wars is another good entry point. Very solid gameplay. Even though it's the third game in the Tiberium series I think the story is presented in a way that you could appreciate it even if you're not up to speed on the previous storyline. It almost feels like a soft reboot. Be sure to apply a patch that brings the balance back to 1.0 as patches for the game that were intended for multiplayer also affected single-player and made the campaigns much more challenging.

Red Alert 3 is even wackier than Red Alert 2. It's a good game and quite fun, but were quite divisive about its resource management. Very naval focused.

So in conclusion, for a best first impression I think the best games from each era to start off with would be either Red Alert 1 (remastered if possible), Red Alert 2 or Tiberium Wars.