I would say go for Realms of Arkania Shadows over Riva as opposed to the first two if you are short on time (the first one is so-so anyhow). My reasons are as follows:
- involving 'intra-party' storytelling: each individual in your party occasionally interrupts with their own input or observations as you travel, each will comment on certain things you try to do (with chunky written descriptions of their attempts to force open a door, use a crowbar or magic etc.). To give a simple example, if you wander around with no clothes on your leading character might receive some angry remarks, and as you walk the streets someone might try to grope another female part member etc.
- a large number of 'random events' throughout your journeys, which makes travelling seem less monotonous.
- relatively superior graphics than Ishar, and on par with M&M (although this is arguable and I tend not to place too much weight on it).
- deep and complex system which allows perfect modelling of character, personality etc. Characters can be afraid of heights (and refuse to cross bridges with you), one can be claustraphobic, one may be seductive etc. Admittedly, not all attributes affect gameplay seriously, but it adds to role-play if you can have one of your group dance for money in the taverns.
- strong party mechanics: in combat (tactical, turn based) and in adventuring (you can split up your party, have one fellow scout ahead or other). When you rest you must assign guard duties et al. (so it really feels like you are micromanaging several people, as opposed to 1 multi-faceted, many-faced 'being')
As for the others:
Ishar: I never really got into this. Dialogues were confusing. One positive aspect is that intra-party politics is modelled, so party members can oppose you should you wish to kick one out (etc.) but other than this I wasn't too impressed.
Stonekeep: not very involving, and a tedious hack-and-slash battle system. Graphics are animated sprites converted from real photos, but they look too childish for my tastes. Not very absorbing really, and terribly, terribly cliche dialogue.
Might and Magic: reasonable. I hated the real-time combat option, but you are free to play turn based. Only issue is it doesn't feel too much like you're controlling a party, just a collective mass capable of shooting arrows and fireballs etc...