what is the best gundam?

[Battousai The Manslayer]

I would say the best Gundam when it comes to battles is either the Strike Freedom, Legend, or the Infinite Justice.

[PK-Sora]

Amen to that, though I always thought the Hi-Nu Gundam was better designed that the Nu Gundam.

Personally, some of my favorite designs include
The FA-010 Limited Production Full Armor Double Zeta Gundam (Sentinel Gundam)
RX-104 Penelope (Hathaway's Flash)
"Hyper" Zeta Gundam (Kazumi Fujita variation)
And of course the Turn A Gundam, which is pretty much a doomsday device.
In terms of power, this is what I consider to be the top 5 Gundams
1. Turn A Gundam
2. V2 Assault Buster
3. Double X Gundam
4. Wing Gundam 0
5. Xi Gundam

Those are some good Gundam but If you want mobile suit I would have to go with the Geloog Jager Shin matusaga Custom.