You mean on type, variation or accessories...

Type: Zaku 1, 2, 2 (2.0)
Variation: Ground, Space Type
Accessories: Bazooka, Normal Rifle, Heat Hawk

I think any zaku variations which is 2.0 (Char's, Shinmatsunaga, Johnny Ridden, BlackTristars, WhiteOgre, MassProduction) are the best rite (Articulations should be superb)? I do owned special variation Hi-Zack but its not 2.0 and I only liked the colour scheme and the uniqueness of it. Posebility is very limited for this model.  Personally I liked the space type better (thrusters ) and always want to have in the future, The White Wolf...

Zaku 2.0, no doubt. But which 2.0 depends on what accessories you want.

Johnny Ridden's R2A looks most badass but the massive leg boosters severely restrict knee movements. Comes with the largest calibre bazooka.

Black Tristar/Shin's R1A gives you big leg boosters and a larger backpack+bazooka, but for the same price you are better off with Johnny.

J type gets leg missile pods.

Char's S type gives you Char's badassery and a funky (but useless) lift.

F type gets a door knob. And two choices between a regular 280mm bazooka or a larger 320mm; although you only have 1 trigger hence you can't use both.

Minelayer is a waste of money. The only thing extra is a larger backpack, which really doesn't justify the higher price.

Zaku Cannon has an awesome backpack but looks radically different.

Alternatively you could also go for the Zeon Zaku F2 which is older, but comes with missiles, rockets, clips, rifle, bazooka... the whole lot. But in typical shitty Bandai fashion, you only get one rifle choice (Zeon Zaku = 90mm machine gun, EFSF Zaku = Zaku machine gun). Both versions in HGUC gives you two rifles but you don't get a bazooka. 

Stay away from the ancient ver1.0 Zaku IIs. Not worth it when there are better choices as described above.
As for Zaku I, you're better off getting the HGUC version. Smaller yes, but cheaper and has better articulation.

Now go and buy some monoeyes.

if favourite MG monoeye my answer would be Sinanju/Sazabi/Kampfer...lol...for Zaku I think my favourite is Black Tristar Zaku II v2.0 simply because of the bulkiness and the unique colours...

this is why we need sazabi 2.0. so that sinanju can have an equally cool friend .