Yes... it's time for some Gundam rant!

Red Z.A.K.U Zeon Char Custom ( and other Char Customs )

Generally , it's not the mobile suit in question which had made the mobile suit a living legend - it was the pilot.
Char has provided a bit of a headache every now and then to the hero with his custom mobile suits ( For one reason or the other - it was usually painted red ) and despite it being an inferior suit , he managed to fight on even ground against the first Gundam.

Hyaku Shiki

Yet another Mobile Suit under the service of Char Aznable - need I say more?

Qubeley Series

The shape and design of the suit is in itself a form of intrigue ( It looks like a... bee? )
As suggested by the term "series" , there are various other forms of Qubeley such as the Mk II and the Elpeo Custom.
Either way , the Qubeley DOES look like a bee...

Quin Mantha / Mansa Queen

Big , bad and definitely dangerous - but it , too , looks like a bee.
A friggin' oversized bee with large laser BEAMS!

Nu Gundam

The strongest impression that this Gundam have left me with is it's Fin Funnels.
The Fin Funnels of the Nu Gundam can be either used as a weapon or be used to form a triangular shield to defend it against smaller projectiles ( Though larger cannons would disassemble the shield... )
I'd like to add Sazabi as well but that may be too much CHAR for the sanity of the viewers.

Zeta White Unicorn

Whilst it doesn't actually appear in the main series , it's appearances in games and various other franchises alike was striking.
It's appearance is exactly of that of the Zeta Gundam - but it is painted white with pink trimmings here and there. When you use it in certain games - you'd feel like a white marauder sweeping across the battlefield.
Not to mention the Zeta White Unicorn is stronger as compared to the older Zeta.

Hi - Nu Gundam

Yet another suit which doesn't exist in the main plotline.
The Hi-Nu , however , does appear in Gundam games and it's abilities in such games were exceptional.
A powerful Funnel system , the arsenal is upgraded from that of the Nu Gundam and it's mobility have earned it a place in the Gundam games.
What more can you ask for?

Psycho Gundam

( This is Mk II by the way )
The U.C series is loaded with oversized Mobile Armor particularly around the Zeta timeline :X
Like the other Mobile Armor of it's time , it's equipped with heavy firepower and arnaments but unlike the others - it's appearance is closer to that of a Gundam compared to the others.
Though a normal Gundam doesn't have a gigantic blades lodged onto it's arms nor does a Gundam fires bullets from it's fingers.
Ouch.