DQ1: Simple, dated, short. No party system, all combat is pure 1v1 affairs. Game is about gold farming and leveling your hero between very distinct map zones. Charming little world and character art tie it all together though. Very little depth to the proceedings aside from a few scant easter eggs.
DQ2: Like DQ1 but bigger, including a brand new 3-man party system. Essentially the same experience though, labyrinthine mazes and gold/experience farming are the most memorable assest of gameplay. The lategame is a tedious mix of backtracking and guesswork as you near blindly search for the next objective, I hated that.
DQ3: A pretty overhauled take on the formula. Job system introduced, sidequests, minigames, etc... Massive quest that contributes much to the meager plot of DQ1-2. Many consider it still the best in the franchise, there may be some truth in that...but I would disagree.
DQ4: Segregated chapters alter gameflow and break up monotony of plot progression. Control a laundry list of characters and heroes through unique troubles until the final act when the whole shebang ties itself together. One of the most interesting games in the franchise, certainly the most personable in the series yet. Crying shame we never got the remake.
DQ5: CATCH EM ALL!! Umm yea. Great characters, great plotline, unique monster capture thing that allows you to fight side by side finally with the slimes you spent hundreds of hours killing in the first 4 games. Regardless of that, the game serves up one awesome quest filled with rich environments and personalities. Though I have not finished this myself, I became smitten with the PS2 remake and it very well could be my personal favorite DQ game(behind DQ4).
DQ6: I've spent the least amount of time with this game overall. Only a few hours really. Graphics are some of the best on SNES, but I really don't know too much about it. Shouta...?
DQ7: Sucks. Very dry characters and plotline. The quest is massive(Finished in 90 hours earlier this March), and the game gives you plenty to do while levelling and upgrading. The dungeons are completely hit or miss, the backtracking irritable, the job system tactless, and the graphics subpar. It's not a disaster, there is still that DQ charm to be found in the richness of the NPC blather, orchestral music, and monster art. Rebuilding a broken world can evoke curiosity, but aside from DQ1's dated mechanics, this is my least favorite in the franchise.