The longest running character I had was a Mystic. I really liked the Mentalist spell lists. Lots of flexibility between attack, defense and utility spells. Essesnce casters can really lay down a beating, but they don't seem to do much else. Note, that is purely my opinion. I have no interest in number crunching mage classes in Role Master. For non-spell caster I briefly played a Warrior focused on longbow. Piercing criticals can really fuck someone's day up.

My most successful rolemaster character has been a dwarf cleric.  A very efficient mix of melee and magic.  This was despite some rather . . . borderline . . . initial rolls (including an appearance of 03, averaging 3d100).  

I have always wanted to make a highly effective arcane caster (magician or sorcerer) but I have never quite made it happen

I played a Montebanc and saw a Duelist played in a Rolemaster campaign that both worked pretty well.
In retrospect my Montebanc's success had more to do with the fact that he looked like Chad Douglas (don't google that at work), his colourful personality, and his upper class background.

So my final answer is Duelist.

My experience is with RM2.

My favorites were the Fighter and the Sorcerer.

If you want to cause damage in RMSS a Mage or Arcanist is the way to go.  Nobody else can hold a candle to them.  If you want a killer party combo get a Mage or Arcanist and a Wizard (metamagic specialist).

If you want to destroy stuff and have access to healing and attack magic go for a sorcerer.  But don't expect to be as devastating as a Mage because you never will be.

Elemental Specialists fall under Mage in case you were wondering.

If you want to win fights play a Mentalist.  Jolts (a first level spell that causes 1 round stun / 10 rr fail) has won more fights than any other spell in the game.  And it just gets scarier from there on in.

If you want to mess with people Mystics are better than Illusionists but Illusionists are good as well and can pick up a Mage list or two cheaper than a Mystic.  Shock bolt is a good Mage spell choice for an Illusionist as it's only level two and highly effective against foes in metal armour.  It also inflicts a great deal of stunning, which is always nice.

Healers are best at healing but a Cleric or Sorcerer is a more effective party member.  Of course, a Healer with some evil channelling lists doesn't have to be a rear eschelon support character.

Rogue (RM2).