CR1 (N64) - strange flaw of not letting you change your robo during story mode, only other parts (gun/bomb/pod/legs)
CRV2(N64) - The longest game in the series by far. Really emphasizes the series staples of Aerial Builds dominating over anyone playing on the ground, explosions are physically huge and highly damaging and you can almost do more damage using only secondary weapons and ignoring the main guns entirely.
CRGX(GBA) - 2D game that is almost a spinoff with how different the gameplay is. The menu based world exploration (ala Shadows of Valentia) is kind of a positive in retrospect to speed things up rather than the usual wandering around. *It is vaguely a twinstick shooter with freeflight in a 1v1 box&obstacle arena setup*
CR:BR(GCN) - First to get US release, insanely tutorial heavy. Has a badly paced story that is really funny for 3/4 and then awkwardly tries to transition to serious post-apocalpse. Gameplay is scaled toned down from V2 (8 Robo types instead of 12 + 3models per type instead of 5, global accleration decrease for all movement)
CR:Arena(DS) - Swansong of series includes minor throwbacks/cameos to all 4 previous games. Gets to 13 Robo types, brings the speed of N64 back... but goes out of its way to heavily handedly nerf aerial robots with the few new parts... unfortunatley they are SO oppresive that they are virtually the best anti-air AND anti-ground, arguably the stalest game in the franchise for human competetion.
(Most USA fans only play GCN/DS CR games)
The original series creators did make Synaptic Drive for switch, which was a non IP with "inspired "gameplay, but A: no story mode, B:for every good new feature there were two bad ones C : online only fighting game with switch netplay.
Story - I think the kid-centric story of (all games but GCN) is the way to go
Aesthetic : Living toy/action figure - (all game but GCN) but can be taken further with things like Capcom's Gotcha Force as inspiration.
Gameplay - Structured 1000 HP vs 1000HP (6knockdown fights) , Mostly going with Arena as a base with the following additinons - From Synaptic Drive - Ground Dashes and Wires - From the fangame Cyberspace Girls - a 3D implemenation of the CRGX only robot (Misty Mirage) with his walk through walls gimmick. and the following hard nerfs - Umbrella Pod, Sky Wave Pod, Gravity Gun, Smash Bomb, Charge Bomb