Of the ones you listed, I am familar with the Giotto and the Geo 2.0.
The Geo will give you the algorithms you want. Its a great little dive computer, especially at that price. The screen is a little smaller then the Giotto and they sell the battery and oring as an expensive package but many divers just reuse the same oring for quite awhile before needing to change it. It is more complex to use because it has both watch and DC setting and so many programmable options but after the initial setup, which can be a pain (I don't think the directions are too user friandly and they use a lot of abbreviations), its not too complicated. A quick review before each trip should suffice. It does have a lot of alarm options that you may or may not want but fortunately, most of then are optional so you can leave them off if you prefer.
One thing I do not like, and it is true of all Oceanic DC's that I have used, they do not hold your nitrox setting more then 24 hours so make sure you are in the habit of checking before each dive. Its a good habit to have anyway.
The Giotto is a great DC too but some divers find the algorithm too conservative. It works for me but then I have a tendency to rash so I dive conservative even with my Petrel. The screen is a little larger and I think more user friendly then the Geo, at least for initial setup. It doesn't have a lot of alarms but the ones it has can not be silenced. It will hold your nitrox setting until you change it.
Both computers have more functions then you are likely to need. Both have 2 gas dive options that you do not need. They also have deep stop options.
Ultimitely, the algorithm may be your deciding factor between the two. That consideration taken alone favors the Geo.
Edit: Just checked the algorthmn of the Mares Smart. It uses the same RGBM as the Giotto and I see no advantage over the Geo so again, looks like the Geo would be the best match for you of the three.