@satchmoz BOOM architecture would be the most likely candidate, as I believe it makes use of speculative execution (it's a superscalar processor with branch prediction). But, I'm not sure.
I'm almost certain that the Rocket core and designs derived from it will not employ speculative instruction execution, so at least for these cores, RISC-V would have a high probability of being immune from Spectre-class attacks.
@satchmoz My own core design, a sub-microcontroller-class processor without any pipelines or caches, is definitely immune. So would PicoRV32 for the same reasons (lack of pipeline/caches, so no need for speculation).