Magic (as always in D&D) is extremely powerful in Baldur's Gate. However you get the highest damage per second in melee (with all kinds of magical buffs in place) rather than in direct damage spells, and a pure mage can often find it difficult to get good enough at hitting stuff, also they're highly restricted in which items they may use to do so.
Fighters get really good at hitting things, but since you're playing solo you'll need a lot more versatility than that, if you want to have an easy time getting through the game.
Thieves aren't really essential as you can just man up and stumble through (or avoid) most traps. Mages can also open locks. Pickpocketing isn't really essential either. Being able to do really high damage in a single attack however, and hiding just by running around the corner, are both well suited for a solo run. Not essential, but handy. The high level ability that allows using any item is also not to be underestimated.
Clerics have some amazing buffs, but a lot of their function works better with a party, though a cleric thief can buff himself for some really grotesque back-stabbing damage - but many of the tougher battles have enemies immune to back-stab, so probably not viable for a comfortable solo run.
Kensai/Thief or Kensai/Mage duals have the problem that you have to be a kensai for a long time before you can use Use Any Item (armors) or spells (Stone skin and project image) to be really safe.
Fighter/Mage/Thief multiclass has the disadvantage of slow leveling (which doesn't apply because you don't have to share the experience points with 5 other party members, thus you'll still level up rather fast). If there's a level cap this matters much more, because you'd be limited to lower levels than single or dual classes. If you play with Throne of Bhaal installed, or if you play BG2:EE, then this isn't as much a concern, as you'll still be able to get high enough levels to get High Level Abilities (such as use any item).
However, Fighters can already use many items, and if you don't mind resting for every few locked boxes you find a mage can do that part of the job. So as said before whether or not you want to include the thief is a question of personal play-style/preference.
Gnome have high Intelligence and allows for illusionist instead of mage (which is cool because you get more spells per level, which somewhat makes up for leveling slower) however you'd loose access to some really cool spells.
Elves get bonus chance to hit with swords and bows.
Alternatively a bard (blade kit) is basically all that in one, except he's a jack of all trades, master of none (except tanking, blades are amazing tanks) - while the F/M/T can arguably become a master at all 3 things given enough XP. An advantage of the blade is that spells based on caster level are slightly more powerful as the blade levels up like a rogue (i.e. fast). So spells like skull traps are really powerful soon. However most spells have caps on how far they benefit from higher caster level - and bards can't get the higher level spells (such as time stop). While they can also have time stop traps and pickpocketing they have little to offer that a F/M/T doesn't have (except for the blade-spin abilities which are handy for increasing AC or attack speed).
You have many options but since you specified easy I'd suggest going F/M/T.