The class selection will have a larger impact on your experience in MT than race. Your role in the group, the ease or difficulty of solo playing, etc. are all determined by the choice of your two characters. Consequently, it is of the utmost importance to solidify this first. You can find our section on classes here.
Once that choice is made, however, if you do not properly choose a race you could easily end up with a sub-par character. Even at higher levels, lacking the proper stats will affect combat as well as HP/MP gain early on. Here are some basic tenets to consider:
Amazon + Feline: Pretty much the only great option. I would choose the sub-race that offers extra chance to dodge
Cleric + High Elf: Clerics tend to be mana-sluts. While this needs balancing to account for their likely acquisition of aggro mid-battle, the high Wis/Int combo of a high elf makes it a likely choice.
Druid + Drow Elf: This choice is made purely for the high CHA of the Drow. The high CHA means the Druid gets to summon the maximum number of charmies.
Necro + Drow Elf: Again, this is for the charmy factor. One of the draw backs of Drow is the low HP gain.
Psionist + Human: This is a balance issue. Psionists draw from all stats and Humans fair well across the board.
Paladin + Dwarf: This selection is primarily helpful in acquiring large amounts of HP. The forte of the Paladin is to absorb damage while self-healing. No race sets up better to acquire high HP than a Dwarf.
On our Medieval Times Top Players page you can find a grid that shows a break down of the most common Class/Race combinations. This can give you a decent overview of the choices that “experienced” players (played 50+ levels) have chosen and perhaps had success with.