Design wise I'd have to go with
5. MSZ-006C1 Zeta Plus C1 (Gundam Sentinel)
4. MSN-04II Nightingale (Char's Counterattack: Beltorchika's Children)
3. MSN-001X Delta Gundam Kai (Mobile Suit Gundam UC (The Postwar))
2. Rx-93-v2 Hi-v Gundam (Char's Counterattack: Beltorchika's Children)
1. XXXG-00W0 Wing Gundam ZERO (EW ver.) (Gundam WING)
But taking all aspects of the mobile suits (history, tech specs, design, pilot, etc.) into account my top 5 would be
5. SYSTEM ∀-99 ∀ Gundam (Black history ver.) (∀ Gundam)
4. CONCEPT-X 6-1-2 Turn X (∀ Gundam)
3. XM-X2 Crossbone Gundam X2 (Crossbone Gundam)
2. EMS-10 Zudah (MS IGLOO)
1. F91 Gundam F91 (Crossbone Gundam)
I've started a Gundam discussion group on Skype, if anyone wants to join.
We discuss all things Gundam, browse /m/ and debate eachother.
Also we play a hilarious game using the Gundam Wiki and the Random page button.
Anyone can join, just pm me or add me on Skype! i_am_antw0n
1) Wing Zero Custom, from Gundam Wing: Endless Waltz
2) Crossbone Gundam X1, from Mobile Suit Crossbone Gundam
3) GM Sniper II, from Mobile Suit Gundam 0080: War in the Pocket
4) Gundam TR-1 [Hazel Owsla] w/ Gigantic Arm Unit, from Advance of Zeta: The Flag of Titans
5) Striker Custom, from Mobile Suit Gundam Katana